Both shows have Angel affect a somewhat inconsistent Irish accent during flashbacks. Notably, in a later episode in which he loses the memories of everything since before he was vamped, the writers had him speaking in American dialect (and being confused by it himself) because his Irish accent had an American accent.
Fred's Texas accent seems to do this in-universe. Amy Acker is Texan and was happy to use her native accent occasionally (when Fred gets excited her accent tends to slip, which is Truth in Television for anyone who's ever tried to lose or temper their accent.) While impersonating Fred, though, Illyria went all out with y'alls and aint's, to the point where it almost seemed hilariously fake.
In first season Angel, Irish actor Glenn Quinn was accused of having a poor Irish accent as Doyle (no doubt because he had a real working-class Dublin accent rather than a "stage-Irish" one). In reality, he was asked to affect an American accent on words the editors thought were difficult to understand when spoken in his own accent.
Buffy Season Three's "The Prom" has Wesley's usually-excellent accent crack on "Thanks for that" when Giles tells him to ask Cordelia to dance.
"Teacher's Pet" in season one features a person-sized "she-mantis" played by a South African, but speaking with an American accent. For the most part, her accent is impeccable and it's difficult to say where she gets it wrong, but there's something about her r's. Also, her vowels.
Then there is Spike. James Marsters does a passable British accent most of the time, but as his presence increases, there are more than a few slips (too round r's, most noticeably). It's worse in the early seasons.
Kendra, played by Bianca Lawson. Supposedly the accent was a last-minute addition, and the dialect coach taught Bianca Lawson an accent from a very specific, obscure area of Jamaica. To both viewers and crew, it just sounded like a lame Jamaican accent.
Arrested Development: Portia de Rossi's American accent slips into her native Australian several times per episode, which is made more noticeable by the fact that her on-screen siblings are all played by Americans.
As many of the actors were British, there was a chance for accents to slip. In episode 3, "Carentan", one can hear Rick Warden's (who plays Harry Welsh) natural English accent slip through quite noticeably on several occasions.
Damian Lewis can be heard using the British pronunciation of "lieutenant."
In the episode "The Son Also Rises", Baltar's defense lawyer, played by London-born Irish/German actor Mark Sheppard, fades between the character's vaguely American accent and something like the "London Irish" accent.
Jamie Bamber, who plays Lee "Apollo" Adama with a pseudo-American accent, slips up occasionally in the mini-series, presumably before he had more time to practice. It holds together pretty well for the rest of the show, though he slips up and says, "Chamallar extract," adding in an "r" sound after "Chamalla," which is a British-ism. (It's also a Boston-ism, but then his pseudo-American accent is pseudo-"General American," not pseudo-Boston Brahmin.)
Tahmoh Penikett slips into his own Aboriginal/Inuit accent regularly in the first season and a half. In the episode "The Farm" it's especially pronounced for some reason; his "Watchya doin' thair Staarbuck" is a particularly good example of a Northern Native cadence.
Michael Trucco falls into this a lot; he was born in Canada and moved to the US as a teen. His accent tends to bounce back and forth across the border, especially as he spent more time on the show. He would even pronounce words in different ways within the same episodes.
Better Off Ted: Portia de Rossi occasionally strays into her Australian accent.
Sheldon Cooper displays an in-universe version; he once briefly reverted back to his natural accent (East Texas) after being locked out of his apartment. He's done it a couple of times since then, usually when he's upset or excited.
Claudia Black: Fortunately, her most notableroles allow her to use her natural accent (which is a pleasant sounding but unusual mix of Australian and British that people usually assume is a bad attempt at one of those accents anyway). In her guest appearances on American TV shows (Hercules, Xena, NCIS, etc.) she tries to use an American accent which always slips near the end of sentences.
Blake's 7: In the episode Time Squad, Roj Blake's BBC English slips into Gareth Thomas' natural Welsh accent when saying "Very diplomatic" to Zen.
Bloodline: Jacinda Barrett's Australian accent slips in almost every emotional scene that the very Southern Diana Rayburn has.
Justified in Boston Legal. Saffron Burrows' American accent slips a lot, but it turns out her character Lorraine Weller is in fact an English woman pretending to be American. It's unknown whether this was the plan all along or if the writers were adapting as in the Burn Notice example further down.
Brookside: There was an actress in this now-defunct Scouse soap opera who was meant to be from a geographically distinct part of Northern Ireland. Part of the joy of listening to the Dublin-born actress (Barbara Dreman) who played Niamh Musgrove was the way, like an Irish traveler on the road, her accent moved and wavered between the six counties of Northern Ireland without once settling on any one, often in the same line of dialogue. (Again, the Jane Leeves thing of mastering a regional accent from your own country which is not your own.)
Burn Notice: Meet Fiona Glenanne, the former Irish terrorist who can't hold an Irish accent to save her life. Gabrielle Anwar is otherwise fantastic, so after the pilot, they gave her an American accent along with the explanation "I can't very well be talking like a freakin' leprechaun now, can I?" Her Fake American accent is much better, but her natural British accent still slips through from time to time.
The NBC series Camp, which takes place in an American summer camp but was filmed entirely in Australia with Australian actors, has received some criticism for a lot of accent slip-ups.
Nathan Fillion and Stana Katic both drop a little Canadian raising here and there, despite playing born-and-bred New Yorkers.
One episode of has an In-Universe example — a self-help guru who's a Harvard MBA from California but affects a Laotian accent and pretends to be an immigrant because "people want that Horatio Alger crap". When they start grilling him over a murder, he gets flustered and the accent disappears.
"Almost Famous" has a male stripper named Hans von Manschaft, who speaks with a German accent which he drops immediately upon learning his rival's been murdered. Castle immediately hangs that lampshade.
In the episode where the actors played not only their normal characters but also other people in the 1940s, Fillion's hard-boiled New York City detective has an accent that can vaguely be described as coming from the Bronx neighborhood of Oklahoma as heard in Alberta.
Angela Cartwright: Despite her moving to the U.S. as a very young child, this British-born actor (Make Room For Daddy, Lost in Space) never managed to consistently erase faint traces of British vowel sounds from her dialogue.
In one episode, the girls are tasked with protecting a collection of priceless jade. Cheryl Ladd's character gets to pose as the jade's aristocratic Swedish owner, complete with a hilariously terrible accent that has to be heard to be believed. ("Ah love-ah mah yade-ah.")
Kate Jackson's French accent, heard in a few episodes, isn't much better.
Their accents, in general, are hard to distinguish from one another as the change in time has made things different and what was a typical Hollywood accent has changed since then, as have the ones they were faking. They are, however, ridiculously overdone.
It was Julian McMahon's first role that required him to use an American accent, and as such his native New South Wales(Australian) accent breaks through a few times in the early Cole episodes. It tends to happen on the last word or two of his lines.
Mark Sheppard appears in one episode as a demon called Arnon. He puts on an American accent but his natural voice slips through when he's angry.
Cheers: The character Cliff Clavin originally had a very broad Boston accent; over time, the actor John Ratzenberger let it fade out, reverting to his normal speaking voice. The only remnant is the accent on his catchphrase, "Here's a little known fact..." It's odd, however, that Cliff is the only member of Cheers who speaks with a Boston accent!
Playing Britain's Prime Minister in COBRA, Robert Carlyle occasionally lets some of his native Glasgow vowels slip through his English accent.
Cold Case: Jaime Bamber's British accent slips through his American one during a "highly emotional scene."
Copper: An In-Universe example in one episode: A Confederate spy is masquerading as a government representative from Ottawa, and speaking with a French Canadian accent, but slips and says "y'all", which exposes him as a being from the South.
Criminal Minds: Jayne Atkinson was originally born in England but moved to the U.S. (first in the Miami area) when she was nine. Her own accent is thus a hybrid of the English and American accents, but on the show, she goes for a more authoritative baritone (to more closely mimic Thomas Gibson's portrayal of Hotch), which she doesn't always hold.
One episode has Roger Daltrey playing a long-missing mob boss who's come back to kill his old crew. For most of his appearance, he affects a pretty convincing American accent... but then the character has a heart attack, and after that, he's pretty much speaking in his native accent.
Louise Lombard's British accent slips out quite frequently as Sofia Curtis. You have to wonder why the producers didn't bother just making her character British.
There's an episode with a supposed South African whose accent was apparently atrocious all the way through.
CSI: Miami: Jamie Bamber's British accent slips into his American.
Cult: Stars Robert Knepper, and he's doing basically the same accent he used on Heroes.
In the first season, Charlie Cox's natural English accent sometimes pushes through the New York accent he puts on to play Matt Murdock in much of the first episode. Over time, he's actually developed a bit of the New York accent into his normal voice due to having settled down in New York City proper.
In select scenes, James Wesley sometimes has a hard time masking Toby Leonard Moore's native Australian accent.
Dark Matter: Melanie Liburd puts on a good American/Canadian/whatever that is in the future accent, but her native English accent slips through occasionally.
Al Swearengen's accent waxes and wanes like the moon. However, because the producers were not confident that Ian McShane could maintain a convincing American accent, they wrote in a backstory in which he was born and briefly lived in Manchester (nearby McShane's native Lancashire). The real historical figure was born in America.
In contrast, Paula Malcomson who plays Trixie is originally from Northern Ireland but her accent hardly slips at all.
Defiance: New Zealander Grant Bolan usually does a passable job playing the American Joshua Nolan, but every once in a while, he has the usual problem actors from Down Under have with their vowels. It's particularly noticeable when he says the word "home" as in the second season episode "Slouching Towards Bethlehem."
Marco's parents, who had quite possibly the most stereotypical Italian accents ever. It slipped in the episode where Marco came out to his father and his father talked with a regular Canadian/English voice. Surprisingly, this actually worked and made the scene a lot better.
Almost everyone on the show inexplicably tries to minimize their Canadian raising, particularly in later seasons.
Becky and Luke Baker, supposedly from Florida, still pronounce their "sorries" and "abouts" like Canadians.
Dylan Everett's came through "load" and clear in a few scenes, perhaps to emphasize his character's a Country Mouse from northern Ontario, far from home in Toronto.
Philip Glenister's American accent in the British show Demons somewhat inverts this trope. It's technically correctvowel sounds right, and rhotic where it should bebut to an American ear it sounds a little too perfect: there's no hint of any regional variation, nor is there any of the inflection or intonations a native speaker usually has ... it sounds a little monotonous, actually, sort of a generic American accent that no American actually has. The writers also seem to have purposely given him a little help by mostly avoiding, in his dialogue, not only any distinctly American words or turns of phrase but even writing around the situations where he'd have to use either them or their British equivalent. Later on the series, there are some scenes where this trope actually happens and you can tell he's actually British.
Devs: Sonoya Mizuno's native English accent doesn't so much slip out as her American accent just seems to change from line and line and generally sounds too much like someone trying to do an American accent.
The show is famous for combining British dialogue and body language with (often bad) American accents in its American characters. For a new series example, see "Dalek". For a classic example, "The Gunfighters". With that said, since Doctor Who is one of the few shows that ever needs to attempt American characters, it might just be fair revenge for the frequent American attempts at British accents...
Dodo was originally intended to have a Cockney accent. The actress could do Mancunian. Executive Meddling decided her having a regional accent was too spicy for the viewers and ordered her to drop it. One Expanded Universe book explains this as Dodo growing up in an upper-middle-class family but with working-class friends, and developing both an upper and lower-class accent between which she naturally switches.
Dodo's first full story, "The Ark", manages to be an inversion because Jackie Lane actually slips back from full RP into Northern vowel sounds in various places.
The First/Second Doctor's companion Polly speaks in over-pronounced RP at first, which soon relaxes to a much more neutral Southern accent. This was because Anneke Wills was asked to do the accent because her character was posh, but hated doing the accent, finding it a bit too obvious, and so consciously scaled it back over time. This does overlap nicely with her character development and the influence Ben has on her.
Jamie was originally played with a quite thick and accurate Highland accent as he was supposed to be a one-shot guest character, but Frazer Hines worried about how intelligible it would be overseas and found it obscured most of the acting nuance. By the time of Jamie's Day In The Limelight, Hines had scaled it back to mild generic-Scots affectations that Hines referred to as "TV Scots". Notice that after Jamie's (second, real) memory wipe in "The War Games", his accent returns to how it was in his first appearance.
John Levene, who played the Second/Third/Fourth Doctor companion Benton, was The Other Marty for Benton's original actor and had never done a speaking part before. He didn't want to speak in his natural broad Salisbury accent for the part and affected a light Northern accent (placed by the Expanded Universe in Bolton), but you can still hear him battling valiantly with his West Country rhoticism in "The Invasion" (particularly the word "car").
Ogron: We - Found - And - Destroyed - The - En-e-my. Commander: Any complications? Ogron:[In mumbly, naturalistic tone] No complications.
Tom Baker's Doctor sticks to a playful take on RP despite being played by a working-class actor from Liverpool. (This was lampshaded by Jon Culshaw while interviewing him in character as him — he pointed out that there isn't the slightest touch of Scouse in his accent and then speaks a sentence with lots of forced Scouse slang in his perfect impression of Baker's voice. Baker responded by dropping into his virtually never observed Scouse accent for a few sentences, correcting the slang.) Baker is from a generation where actors were strongly encouraged to drop regional accents in favor of RP, and so speaks in an RP accent naturally, if more neutral and much less ornamented than the voice he uses for the Doctor. The Liverpool is still occasionally audible, though, especially in Season 17-18 when he was having health problems and was burned out with the role.
In a similar example to the "Judoon platoon upon the Moon" line in the Tennant section, note the scene in "The Ark in Space" which makes a point of making the Doctor repeat the phrase "red, yellow, green", because the word green is difficult for Baker to say in the accent he gives the Doctor. It comes out as a strained "red, yellow, gnuiin" that gets funnier every time he does it.
In "The Horns of Nimon", there's a flub where he starts addressing K9 in his higher-pitched and more Northern natural voice for a few words, before catching himself and restarting the line in his usual acting-voice.
Tom uses a glaringly Scouse pronunciation of the phrase "certain 'erbs" in "State of Decay".
A lot of this can be explained away in-universe by the detail that Baker's Doctor is also kind of nuts, meaning that faking accents, bouncing between pronunciations and the like probably doesn't even rank in the top ten weirdest things he's done at random.
One of the Fifth and Sixth Doctor's companions, Peri Brown, was supposed to be from Pasadena, California. However, her actress, Nicola Bryant, a native Briton, apparently couldn't pin down a California accent if she tried and instead took Brits on a linguistic tour of the United States with each serial. It didn't help authenticity that the scripts she was given didn't use American terms like "truck" or "elevator" in favor of British ones like "lorry" and "lift" because The BBC thought it'd confuse the audience.note Given that during her stint, the BBC ran Dallas, DynastyandThe Dukes of Hazzard it is unlikely viewers would've been confused.
Captain Jack Harkness, while not American (he's described as being from the Boeshane Peninsula in the 51st Century), is stated on the show as having an American accent. John Barrowman, who plays Captain Jack Harkness, is Scottish but spent some of his childhood in America. He cultivated an "American" accent that while wildly applauded as "Mid-Atlantic" in Britain doesn't work as well with actual Americans. This is made worse by the British English in his scripts and the occasional slip back to his native Scottish accent. Depending on whom you talk to, "mid-Atlantic" might mean anywhere from Delaware (the actual meaning used by linguists) to Rhode Island (where Barrowman lived in America; an accent most often heard from Peter Griffin). Barrowman himself is much more modest about his accent than his fans, describing it as a hybrid he developed as a child to avoid bullying.
David Tennant, although rarely, does slip into his Scottish accent a few times, mostly on the word "creature".
The writers intentionally had some fun with this in "Smith and Jones". The syllable "-oon" is particularly hard for him to say without sounding Scottish, so they gave him the line "Judoon platoon upon the Moon".
During "The Impossible Planet" when they walk through the corridors and he says the line about how the base was built. It's almost entirely Scottish.
"The Stolen Earth" has a scene where the Doctor speaks un-translated Judoon, something Tennant found nearly impossible to do in the Doctor's English accent, and which was specifically put in just to screw with him.
If you listen closely to the Doctor and Wilf saying goodbye in "Journey's End", half of what David says is noticeably Scottish. Particularly the lines "They've all got someone else" and "You can never tell her".
In "Tooth and Claw", since the Doctor and Rose arrive in 1879 Scotland, the Doctor adapts a Scottish accent (a little broader than Tennant's native one). When the werewolf shows up and he drops back into the Doctor accent, Queen Victoria catches notice of this. In the initial scripts, Rose was also supposed to be faking a Scottish accent, but Billie Piper was unable to produce a brogue that sounded convincing, so she just dropped it, and instead a scene was written where Rose tries to do an accent, but it's so hilariously terrible the Doctor tells her to cut it off.
His accent drops noticeably towards the end of "Time Crash", such as on the word "trainers". This actually works very well, as it crosses strongly into meta.
In "The Day of the Doctor", the Eleventh Doctor insults the Tenth Doctor by calling him "Dick Van Dyke", whose atrocious Cockney accent in Mary Poppins is a cultural meme in the UK. Eleven goes on to poke fun at Ten's artificial, vain attempts at Britpoppy coolness and how he's going through his "grunge phase", which (since Britpop and UK Grunge were notorious for popularising class-tourist "Mockney" accents amongst the middle class) seems to imply that the Tenth Doctor's accent is an in-character affectation, likely because he thinks it's cool. (The TV chef Jamie Oliver, all the rage in the mid-00s, was one of the figures who inspired the Tenth Doctor; he was a notorious target of mockery at the time for his fake Cockney affectations.)
Tennant also slips into a neutral posh English accent quite a lot during the specials, most notably "The Waters of Mars". He had been playing Hamlet with the Royal Shakespeare Company for months, to the point where Hamlet's posh accent had eclipsed the Doctor's as his default English accent. He claims he had to buy all the DVDs and watch his performance again to relearn how to do it. Of course, during "The Waters of Mars" the Doctor loses his handle on reality and declares himself to be a god, so his accent going up two social classes seems to suit his hubris.
For "Turn Left", Billie Piper apparently lost the accent she had used as Rose in Series 1 and 2, so she speaks with a noticeable lisp. She also has a pronounced overbite that she never used to have, mildly affecting her speech (and possibly causing the lisp). Piper said in an interview that she sounded like that because it was cold as they were shooting in winter and she was wearing a "tiny jacket". On the other hand, some fans note that Rose sounds quite a bit like the Tenth Doctor down to his inflections and speculate that this was intentional to show how much Rose had become like him.
Paul McGann's Eighth Doctor has a subtle case of this. His accent travels a sort of spectrum between Received Pronunciation and Scouse. Sometimes it's more of one, sometimes it's more of the other. And sometimes it's something in the middle which is sexier than either extreme. It's mostly more on the posh side, but note how he pronounces, "I don't believe in ghosts" his accent veers off Oop North.
River Song may be an almost extreme case. Her first few lines are in some strange very light semi-African accent, then never again.
Canton Everett Delaware III, played by Mark Sheppard, is a particularly atrocious example. His American accent is sporadic at best. It would have made much more sense for the producers to use an actual American actor.
While Jenna Coleman uses her Lancashire accent for most versions of Clara Oswald, she uses a cockney accent (and, to a lesser extent, an RP-like one) for the character of Victorian-era Clara. It's a rather good rendition of cockney, but she sometimes slips out of it and you can briefly hear traces of her native accent. Justified in retrospect given that the Victorian Clara is supposed to be an incarnation of the modern-day Clara, so the accent slip may be the original Clara asserting herself.
In "The Stolen Earth", Martha Jones' superior is General Sanchez, supposedly an American. However, while his accent is adequate, his lines and body language are relentlessly, flagrantly British. This is especially true in his last moments when rather than taking cover and firing one-handed, or dropping into the two-handed Weaver stance used by the Americans, he leaves cover and uses a hand-over-hand British technique.
Dollhouse: There's one episode in which Mark Sheppard's accent slips right back to his native British on the line "What happened there, Ballard?".
Paul Blackthorne frequently slips into an English accent in this TV series, especially during the voiceovers.
In an amusing Inverted Trope, Terrence Mann's fake British accent slips occasionally into his native southeast US one. It's most noticeable when he pronounces names like "Lafayette".
Eastenders: Featured the character of Vicki Fowler, an English - born teenager who had grown up in the USA and spoke with a Fake American accent that kept slipping. By the end of her run on the show, the accent was dropped entirely and she spoke in the same Cockney dialect as the rest of the cast.
In Faking It, Gregg Sulkin puts up an admirable attempt at sounding American, but it just won't stick. To be fair, few of the actors playing teens even attempt to sound like native Texans.
The Fall: While Gillian Anderson is probably better positioned to play an English detective than most American actresses, given that she lives in the UK, there are scenes where she hides her native accent about as minimally as Elisabeth Moss does in Top of the Lake, sometimes even going full rhotic. This is odd as she actually grew up in the UK and has been noted before for an ability to put on the accent. Perhaps it has been too long now.
Farscape: During the first season Chiana switches back and forth between a Fake American accent and Gigi Edgley's native Australian one from episode to episode. This was because the directors kept changing their minds as to which accent she should use. Eventually they settled on the American one which she used for the rest of the series.
Father Brown in the 1970s. Actors playing Americans had uneven accents, to say the least. Professor Smaill in "The Curse of the Golden Cross" had some of the most noticeable difficulties.
The Feast Of All Saints: Happens a fair bit in the Showtime miniseries of Anne Rice's story. To get the effect of French Creole characters living in antebellum Louisiana, the cast speaks English peppered with French accents. As with anything else, some of the actors are very good and consistent with the accent, but most slip up at least occasionally. And then there are those who don't even bother trying.
Firefly: Every so often, Jewel Staite (a Canadian from Vancouver) would let a Canadian "ou" (i.e. aboat) slip into her Wild Western accent. Nathan Fillion (also Canadian, from Edmonton) also did this, probably most notable at the beginning of Serenity where Mal is in the cockpit with Wash. Of course, all the characters on the show come from various planets where many dialects of English and other languages have evolved over centuries.
Jane Leeves' fake Mancunian accent is pretty strange to British viewers. Jane Leeves is British but was born in Essex and raised in Sussex — nowhere near Manchester.
None of the actors playing Daphne's brothers - played by a Scot, an Australian and an Englishman born in Swaziland - attempted even an approximation of a Manchester accent. Anthony LaPaglia's accent might charitably be called South London while Robbie Coltrane, when he did speak, sounded like a stereotypical unintelligible Scotsman.
Kelsey Grammer's own natural accent is the classic loose Florida drawl (he is, after all, a Floridian), not Frasier's clipped upper-class tones. He tends to slip when Frasier is supposed to be drunk or tired.
Comedy duo French and Saunders have a running gag where whenever they do a parody of a movie/tv show, they'll lose their accent at some point in the parody (if they bothered with one in the first place). The other one will then say "You're not going to bother with the accent, then?" which prompts the other one to try and get back into the accent by saying "how are you" in a ridiculous way that sounds more like "hay ay yew". Their catch-phrase "how are you?" in these situations is done in a Northern Irish accent. Entertainingly lampshaded in their Star Wars parody where Saunders played Liam Neeson's character. Since Neeson is Northern Irish, for once the accent was right.
Game of Thrones: With the show's main setting, Westeros, being heavily based on Medieval England, any cast member whose character originates from there speaks with a regional accent depending on whereabouts they're from and which area of England is being used as a parallel - Yorkshire accents for Northmen, Received Pronunciation ("Queen's English") for King's landing, etc. While most of the cast that aren't speaking in their natural accents do an admirable job, there are some attempts that those familiar with regional English accents might sound jarring - notably Aiden Gillen as Littlefinger, whose accent jumps from RP to Irish and back again on a sentence-by-sentence basis. And Peter Dinklage, brilliant though he is, speaks in a mangled approximation of a British accent that can barely be pinned to any particular area of the British Isles, though at least this is consistent. Jack Gleeson as Prince Joffrey also occasionally slips into his native Irish brogue. Nikolaj Coster-Waldau's native Danish sometimes slips through as does Richard Madden's Scottish accent and Michelle Fairley's Northern Irish accent. In "The Door", Euron Greyjoy (played by Danish actor Pilou Asbæk) gives a rallying speech as to why he should be named the new leader of the Iron Islands, during which Asbæk's sing-songy Danish accent comes through multiple times.
Get Smart: Early seasons feature Don Adams' using his signature accent (known as "Glicking") far more than in later seasons. Adams commented once that he learned fairly early how little Glicking he could get away with while still getting the idea across.
The Good Fight: Michael Sheen plays a Slimeball lawyer with a thick New York accent. Once you've heard his native Welsh, however, you can hear it slipping through in this accent.
Good Omens (2019): Another David Tennant one. As the demon Crowley, he does the majority of his performance in his Doctor Who English accent (which he also used for Kilgrave in Jessica Jones), but it couldn't quite hold up to yelling at those houseplants. Complicated by the fact that Crowley also in-character puts on a Scottish accent when he's being Nanny Ashtoreth.
Gracepoint: David Tennant yet again, this time playing an American detective who apparently turns Scottish whenever he raises his voice.
Rachael Taylor is a blonde Australian actress playing an American doctor, whose native accent keeps trying to reassert itself, especially when she's speaking very fast in the operating room.
Since 2012, British-born Camilla Luddington has played Jo Wilson. While her American accent is usually very good, it occasionally slips. Noticeable whenever Dr. Wilson has to say 'Dr. Pierce' and it tends to be really obvious that she's English.
Hancock's Half Hour: Parodied in the episode where Hancock's character is a ham actor on a radio soap whose "rustic" accent keeps mutating from Welsh to Cornish to Robert Newton.
In one of the trailers for this new NBC series, the English Hugh Dancy, playing American Will Graham, slips noticeably when shouting 'FBI!'; the 'i' sound comes out more like 'oi'. Apart from that, it mostly holds up.
Hugh Dancy's American accent is very good most of the time, but his native English is notable when he says a few words, particularly "anything" (said "en-eh-thing" rather than the American "en-ee-thing").
Haven: Although the show takes place in Maine, it is shot in Canada, and several of the actors are Canadian, most notably lead actors Lucas Bryant (Nathan) and Adam "Edge" Copeland. While the former attempts to hide his Canadian accent, the latter makes no effort.
Hawaii Five-0: Aussie Alex O'Loughlin usually does very well in maintaining his fake American accent to play Steve McGarrett but there have been some slips, the most noticeable one in Season's 3 "Hookman".
In the first few episodes of season one, Mohinder has an Indian accent which has been replaced by something resembling a British one. Neither of those are the actor's real accent, which is American. Amusingly, Sendhil Ramamurthy had a guest appearance on Psych where, instead of doing the full Mohinder, he tried to go with an accent about halfway between Mohinder's British one and his own natural "California Valley Dude" accent. Instead, it just seems to slip between one and the other throughout the episode from scene to scene.
Whenever Sendhil is using his natural accent, like on Psych, there are people complaining how "his fake American accent" is so unconvincing, and he should stick to "his natural British accent". He's from Texas.
In fact, Sendhil has admitted to doing an intentional Ooh, Me Accent's Slipping on Heroes because the producers changed their minds about what accent he should have after they'd already filmed the pilot. (This would certainly explain why Hayden Panettiere speaks with a slight Texan inflection when playing Claire to begin with (she and the Bennets live in Texas) that soon disappears.) So for the first few episodes, he attempted to gradually transition from "Indian" to "British."
And then there's Robert Knepper, who puts on a truly fascinating trainwreck of an accent which slips around various American and British tones with every other word.
According to Knepper in an interview, he admits that the slip-ups are deliberate to make Samuel more "worldly."
If you're Danish, his last name is a source of never-ending hilarity.
Lisa Lackey (who plays Matt Parkman's wife/ex-wife Janice) is Australian, and her Australian accent slips through on occasion in her scenes with Matt.
Horrible Histories: In this TV adaptation, this often happens on purpose when something unexpected, weird, embarrassing or gruesome happens, such as during the Dick Turpin song, the words "that's lame" are in the actor's normal voice, but the rest of them with an altered voice.
Stars English actor Hugh Laurie as the American title character Dr. Gregory House. His American is so good (if a little creepy) that when his audition tape was viewed, executive producer Bryan Singer, unfamiliar with Laurie's nationality or previous work, singled him out as an example of a "real American actor." Laurie has gone on record that after a long shoot his accent drifts towards something more "French sounding." (Note that he tries to keep his American voice all day long even when he flubs a take).
Laurie does slip up noticeably on one line in the second episode of the show: when talking about a high schooler who potentially has a degenerative brain disease, he says "this kid's gonna be picking up his diploma in diapers and a wheelchair", dropping the R in "diapers" and inserting an R in "diploma", so that it sounds like "diplomer in diapehs"
In an interview, he mentioned that words such as "coronary artery" are extremely difficult for him to pronounce in an American accent. Good job he doesn't have to very... oh, of course.
Similarly, it seemed rather cruel to name an important character "Amber" when a Brit and a New Jerseyian would pronounce those very differently.
Catherine Durant, a Democratic Party Senator, is played by English actress Jayne Atkinson. While during the whole series she does a decent job of faking her accent, she lets go a massively non-rhotic RP English rendition of the word "however" during a convention speech in episode 10 of season 4 (Chapter 49). The fact that this is the single word of her speech that is shown to us through the Live CNN broadcast before abruptly cutting to ordinary camera work could indicate that this may have been a subtle post-production joke.
How I Met Your Mother: Lampshaded in one episode, when Barney Stinson's painfully over-the-top video resume features Barney being interviewed by a voiceover of himself with a very bad English accent. Narrator Barney's accent switches to Scottish before the voiceover is abandoned altogether.
It: Tim Curry is usually so flawless at American accents that if you've only seen him in American roles, you might be forgiven for not knowing he's actually British. He still slips up sometimes, though. As Pennywise the Clown in the made-for-TV film adaptation of the book, he seemed to be going for a "Chicago gangster" sort of voice (or at least a parody of what a Chicago gangster is supposed to sound like) with occasional touches of Bert Lahr (the actor who portrayed the Cowardly Lion in The Wizard of Oz. However, at one point his natural accent shows through slightly. (Since Pennywise is, of course, a vaguely arachnoid, soul-devouring metaphysical freak who must necessarily be a master of disguise, that might or might not have been intentional.)
Jessica Jones features three Australian actors as American characters: Rachael Taylor as Trish Walker, Eka Darville as Malcolm Ducasse, and in season 1, Wil Traval as Will Simpson. Like with Gray's Anatomy, Rachael Taylor has the most slippage when it comes to masking her Australian accent, and it tends to be most noticeable in her scenes with just Simpson or Malcolm.
Stuart Milligan, playing American stage magician Adam Klaus, had an accent that notably veered into English on certain long vowels.
Ditto Anthony Head, in the few times he played the same role.
It was later revealed that Adam Klaus is a Fake American (he's actually Scottish) so this is excusable.
Although Stuart Milligan isn't; he's genuinely Boston-born.
Just Cause: Australian actress Lisa Lackey (see Heroes above) plays Alex DeMonaco, an American from Los Angeles, but she occasionally slips into her native Australian accent or various regional U.S. accents (particularly New York City). This is lampshaded and justified in the pilot when Alex tells Whit she was an army brat and traveled all over growing up, specifically mentioning Australia, New York, and East LA as places she picked up accents from.
In the British legal drama Justice the main character is supposed to be from Liverpool, but the actor's Welsh accent slips in on a semi-regular basis.
On an episode of Late Night, Seth Meyers presented a trailer for "Boston Movie", a hilarious (and let's face it, completely accurate) parody of tropes frequently seen in movies set in Boston, including, "every type of Boston accent, including a British actor trying his best, but not quite making it." Cue a scene of Meyers himself playing said British actor, indeed struggling to suppress his natural accent and sound like a Bostonian.
Legend Of William Tell Beth Allen, playing Vara, and Kieran Hutchinson, playing the titular Will, both New Zealand, do very well generally with occasional slips. Most of the problems seem to be with words like yes and dead, and Will's negotiations. Most of the extras slide closer to Not Even Bothering with the Accent.
Christian Kane plays a cowboy, but his Southern accent goes in and out so many times, it could qualify for a drinking game. There are whole sections of dialogue in which he doesn't have an accent before he remembers that he's supposed to have one and suddenly drops off back in. This gets even stranger once you realize that the actor is from Texas.
Claire's mother, Carole, is portrayed by a British actress whose accent does not pass muster.
Sayid seemed to fall into a strange British accent a lot in the 6th season. It certainly wasn't Naveen Andrew's usual Cockney, but it was also not Sayid's usual voice. Perhaps it has something to do with the fact that he died, and the voice change was intentional.
Alan Dale, who is from New Zealand, plays Charles Widmore, who is English, on Lost, and his accent occasionally slips.
Sun originally speaks with a Korean accent for most of the first two seasons of the show. However, at some point in the series, the accent completely disappears, and Sun inexplicably speaks with a perfect American accent.
MacGyver (1985): English actress June Chadwick fights a losing battle with an American accent as Dr. Ludlum in "The Human Factor."
Australian Simon Baker generally maintains a decent American accent in this show, making it all the more jarring when his "Stay tuned for our next episode" voiceover sounds like "Stay tyuned fer ow' next episode."
While usually Simon Baker has a good American accent he does slip up a lot of words like "talk", "walk", "awkward", "awesome", and similar ones with the same sound.
It also happens when his voice is very low.
Castmate Owain Yeoman (a Welshman) does a very good American accent, but when he loses it, he loses it big. Not just a word here or there but a full sentence sometimes slips.
Once in a while on Series 5, Alexander Vlahos's Welsh accent makes a brief appearance.
So does Colin Morgan's Irish accent.
Poppy Montgomery: She is guilty in Without a Trace and Unforgettable. Also in an episode of Party of Five she plays an English girl, but her Australian accent slips through when she says words like "know".
In one episode, Eric Idle plays a U.S. movie producer ("Scott Of The Antarctic") with a truly painful-to-listen-to Fake American accent.
This goes for everyone from Monty Python any time they try to do an American accent. Except for Terry Gilliam, of course, but he's been in England for so long, he has some British inflections that make his natural voice sound like a fake American accent.
Motherland: Fort Salem: Swedish actress Amalia Holm's American accent is usually flawless, but occasionally a bit of her native intonation will come through.
The Nanny: While Daniel Davis does a truly excellent RP English accent as Niles the Butler throughout the series, there is maybe one word in every three episodes that he elongates a vowel just a little too much or overdoes it just a tiny bit. Such slip-ups were barely noticeable - on both sides of The Pond - but are detectable if you really listen closely and know what you're looking for. That being said, Davis's English accent was so good that some genuine Brits thought he was English and should have given accent coaching to Charles Shaughnessy, who played Maxwell Sheffield - Davis is from Arkansas, while Shaughnessy is from London and a Baron.
Fran Fine and her family speak in stereotypical Noo Yawk accents throughout the series. Given how Fran Drescher has said that her real-life accent isn't as pronounced as it sounds on the show, there are some times where that accent will slip. For example, in "Ode To Barbara Joan", Renee Taylor, who plays Fran's mother Sylvia says "Run and get a butter knife", rather than the stereotypical "buddah".
Nashville: Given that most of the main characters are played by people who aren't from anywhere near Tennessee (with the exception of Kentuckian Will Chasenote Connie Britton is from Massachusetts, Hayden Panettiere and Eric Close are from New York (State), Charles Esten from Pennsylvania, Jonathan Jackson from Florida, Chris Carmack from Washington DC, the Stella sisters are Canadian and Clare Bowen's Australian), this trope does come up occasionally. The most grating example is in one season two episode, where Sam Palladio's usually quite good American accent suddenly decides to have a fight with his regular English voice.
NCIS: New Orleans: Scott Bakula tries to do a Cajun accent. It gets better over time, but especially in the Backdoor Pilot, he would switch between Cajun and his standard American accent every other line (and sometimes in the middle of a line).
Ned's Declassified School Survival Guide: There was a character named One Bite who had a regular American accent. By the next scene, it literally turned into some random Chinese accent. An egregious one at that.
Colin O'Donoghue suffers a few accent slippages since he's Irish but putting on an RP voice for Captain Hook.
Tim Phillips is Australian playing Cinderella's prince with an American accent. It's completely spot-on, except for a brief slip when he's trying to convince her not to run away.
Another Australian - Bernard Curry - plays Captain Hook's brother Liam. His English accent is far less convincing.
The Darling siblings have two non-Brits doing accents. James Immekus slips in a couple of sentences playing Michael. Freya Tingley, however, does a flawless accent to play Wendy.
Robert Carlyle isn't doing an accent but rather toning his Scottish brogue down a little. There are sometimes - particularly when the character is angry - that he slips into a heavier Scots accent.
The Orville: A rather bizarre one as Seth MacFarlane is an American and isn't putting on a different accent: Several times during the show, when trying to sound dramatic, he slips into the voice of Brian the Dog from Family Guy (whom he voice-acts as).
Oz: There's an In-Universe example in this show. John Basil (Lance Reddick) is undercover, going by the name Desmond Mobay and using a Jamaican accent. None of the drug runners in Oz question the accent, but Augustus Hill catches on when it slips for just a second.
PJ Katie's Farm: The main voice actor, PJ Katie, occasionally had her characters randomly gain or lose accents. This is somewhat understandable considering that she did the voices for every single character and everything was done in a single take.
More or less every character after the tenth season Wild Force; though the characters were always American, usually Californian, their actors were predominantly from New Zealand and Australia, and the accents were invariably transient. Until Saban bought back the franchise, the last season in which the Ranger actors were primarily from North America was Power Rangers S.P.D., though Monica May (Z) was the only American; the other actors were Canadian.
In Power Rangers Dino Thunder, Conner's Kiwi accent could reassert itself at a moment's notice, especially if the character had to sound annoyed.
Notably, in Power Rangers Operation Overdrive: "Once A Ranger", the Rangers lose their morphing powers, and this, for some reason, also seems to deprive them of their "American" accents.
Perhaps the most egregious example: Jarrod from Power Rangers Jungle Fury. Every single one of his lines is a struggle between Kiwi and some kind of American hybrid.
The greatest ongoing battle in Power Rangers RPM is "Flynn McAllistair" versus "Scottish accent".
There was a notable case even before production of the series moved to New Zealand with Kat, the second Pink Ranger, who as an Australian was the first non-American Ranger. This trope was averted with the casting of legitimately Australian actress Catherine Sutherland, who used her native accent. When the Rangers were de-aged for several episodes by the villains, however, the American child actress who played the younger Kat couldn't get the accent right despite her best efforts to emulate Sutherland's native accent. She would typically alternate between an overplayed stereotypical Aussie accent and her own native American accent (sometimes going back and forth 3-4 times within a single scene).
Reuben Turner, who plays James Navarro on Power Rangers Dino Charge, would occasionally have to dubbed over by another person due to his Kiwi accent being too thick. Since the character's supposed to be from America like his son, who has no accent, leaving this unchanged would have stuck out like a sore thumb.
Primeval's fourth and fifth seasons were filmed in Ireland instead of Britain, and thus featured a lot of Irish actors pretending to be English - with varying degrees of success. Within the main cast, Ruth Bradley and Ruth Kearney's accents were good - as were some of the submarine workers. Other minor characters, as well as Janice Byrne who played April, slipped quite a bit.
Has an in In-Universe example—a fortune teller speaks with an eastern European accent until things get really serious, and then it totally disappears.
Also on Psych, Liam James (who plays young Shawn in the flashback scenes) sometimes reverts to Canadian raising ("he passed oat.")
Quantico: Priyanka Chopra's Indian accent comes through frequently. The character's mother is Indian, and after she killed her father, Alex was sent to live with her mother's relatives in Mumbai for over a decade. Notably, her Indian accent is a lot stronger when she's highly stressed (being interrogated after the bombing), and is much softer when she isn't as stressed (training at Quantico).
Robert Llewellyn's accent as Kryten (which was meant to be Canadian, but really really isn't) changes to Llewellyn's own Northampton accent in one brief scene in the episode "Polymorph" ("It's here..." — "Where?" — "Somewhere...").
And, after the last of Kryten's "...with just two minor flaws(/drawbacks)..." gags, Danny John-Jules slips out of the Cat's "soul singer"-esque accent into his normal voice when he shouts, "Okay, forget it!!"
Revenge: Elena Satine, an actress from the country of Georgia, plays Louise Ellis, an American from the state of Georgia. She makes a halfhearted attempt at a Southern accent...when she remembers.
Terminator: The Sarah Connor Chronicles: Normally Lena Heady's American accent is flawless, but in exactly one spot - it's very hard to catch - her accent slips at the end of a sentence, during one of Sarah's end of the episode monologues. (This troper can't remember which episode, though - she'll have to rewatch to find it.)
Schitt's Creek: The show is Canadian and the series creators have said the town is somewhere in Canada, although it's never confirmed onscreen. So, when actors who play the townies like Jen Robertson, Dustin Milligan and Noah Reid slip on their o's and start to sound Canadian, it works. However, Alexis Rose seems to have grown up in New York and Los Angeles, but when she says "sorry" the fact that Annie Murphy is Candanian becomes adorably clear.
Phoebe Tonkin (an Australian actress) is playing Faye (a girl living in Washington). In the pilot, at least, it is painfully obvious that she is Australian. She seems to be getting better as the series progresses, though.
Nick's Aussie comes out when Charles is drowning him to kill Abaddon.
seaQuest DSV. Season one's "seaWest" features David McCallum as a sort of quasi-Western sheriff in an underwater Australian mining colony (yes, really). McCallum seems to be trying a broad working-class Australian accent but instead sounds like a Cockney trying and failing to do a bad West Country accent instead.
Shadowhunters - the Seelie Queen is normally played by Lola Flannery who is American doing a convincing English accent. She's able to take on a different form, played by Sarah Hyland, who is also American. Her accent is much less convincing, and she slips on words like 'water'.
Shortland Street: At one point had an American neurosurgeon and one character's British paramour, both of whose actors started slipping into New Zealand accents after about a week.
The Smoke: Jamie Bamber's upper-class accent slips into the working-class accent he affects when playing a firefighter.
Sons of Anarchy: Especially in the latter seasons, Charlie Hunnam's Jax Teller will occasionally slip into Hunnam's English accent. A prime example is when he puts the vote to the club to patch in prospect George "Ratboy" Skogstrom as a full member. "All in favor of welcoming Joh-ahje Skogstrom..."
Stargate SG-1: Though all of the non-alien main characters are American, two of them are played by Canadian actors: Samantha Carter by Amanda Tapping and Daniel Jackson by Michael Shanks. In most cases, they manage to avoid saying any Canadianisms, but occasionally they slip. (Most often this is on the word "sorry.") Cameron Mitchell's accent seems to waver, too, although that's unclear whether that was the fault of the actor or because the writers couldn't decide or agree on where exactly the character was supposed to be from. Ben Browder was raised in Tennessee and North Carolina, but they eventually stated that Mitchell was from Kansas.
In the episode "The Doomsday Machine", James Doohan (Scotty) noticeably lapses into his native Canadian accent at one point. It's the words "Thirty seconds later, blammo" as he's explaining to Kirk how he's wired the Constellation to self-destruct for the climax.
In "Who Mourns for Adonais", Carolyn Palamas sounds pleasantly like Boston (which the actress, Leslie Parrish, is). However, her line "What you're saying makes no sense at all!" is spoken in an attempted British accent.
The Suite Life on Deck: One episode where the group goes to the author of Sherlock Holmes' home, the original draft of the first Holmes story, which is on loan from the Queen's library, is stolen. Cody then tries to figure out who the thief is, and succeeds, as a supposed British detective stole the book, which Cody figured out because his accent slipped, saying 'fries' instead of the more British 'chips', revealing that he is Belgian.
Supergirl (2015) features Irish actress Katie McGrath as Lena Luthor. Her Irish accent is audible under a few words. Fans have theorized that since Lena was adopted, she could be an Irish girl whose accent has simply faded.
Supernatural: Julian Richings - who was born in England but lives in Canada - plays Death with an English accent. At times you can notice a Canadian accent slip in, especially in the season 6 episode "Appointment in Samarra".
The show is filmed in Canada but set in multiple states in the US, so Canadian actors and actresses are more or less a given. And so, Canadian accents are obvious in many episodes.
Tammin Sursok: Poor girl can't catch a break. Since coming to the United States from Australia, where she had both an acting and a singing career, she has only had roles that require an American accent and in nearly every one, she slips up at some point. Usually, it's pretty convincing, but she has trouble with a few sounds and you can really tell she isn't actually from the US. It's most noticeable in Pretty Little Liars and the final season of Hannah Montana.
Switched at Birth: While Katie Leclerc is deaf in Real Life it's both sporadic and due to a condition that developed in her early 20s, so she had to learn the speech impediment that someone who was profoundly deaf since they were a toddler (like her character Daphne) would have. Sometimes this "deaf accent" (her term) slips. (Interestingly, in "Yuletide Fortune Tellers" - involving an alternate reality where the girls weren't switched - Bay-as-Daphne is deaf but speaks with her regular voice throughout, with it being handwaved away via intensive voice therapy.)
Australian actress Cassi Thomson sometimes has trouble keeping her native accent from coming through as Nikki (Toby's ex-wife).
This trope gets averted with British actress Rachel Shenton's character of Lily, who's played with her native accent - Lily is American but her family moved to England when she was young.
Strike (2017): A very interesting example with Deuteragonist Robin Ellacott; while it's accidental on the character's part, it's actually done deliberately by her actress Holliday Grainger as an acting technique. In story 1, in her first scene with fiancee Matthew, Grainger's normal voice is replaced by Robin's childhood Yorkshire accent as she talks to him about her new boss, Cormoran Strike. In story 2, Robin's Yorkshire accent comes out again - only this time in a conversation with Strike about her becoming an investigator and bigger part of the business. It signifies her initial intimacy with Matthew, and later her growing closeness with Strike. It doesn't show up again in her private conversations with Matthew, foreshadowing their growing apart.
Time Trax: This was known to happen in a show set mostly in the US (although at least one episode was set in London), filmed in Australia. (And yes, they had one or two episodes set in Australia.)
While Elisabeth Moss made out generally (and, to some, surprisingly) well with a character whose background called for a hybrid Australian/New Zealand accent, there are nevertheless scenes in which she can just barely conceal her native American accent.
Peter Mullan doesn't seem to be trying to hard to hide his Glaswegian accent in some scenes, particularly when he yells and swears at his sons near the end, either.
Torchwood: James Marsters' British accent slips occasionally as the British Capt. John Hart.
Stephen Moyer occasionally slips into his native British accent in a painfully obvious way.
The same for many of the actors on the show, as most of them are not native to the American South.
While Jessica Hamby begins her tenure on the show with a pretty strong Southern dialect, Deborah Ann Woll (who was born in New York City and attended the USC) gradually loses the Southern accent as the show progresses. By the last season, the dialect is completely gone. Not coincidentally, this is likely because Woll was adjusting her accent for her next role as Karen Page in Daredevil.
Though one actress manages to slip into two different Southern accents and random points.
The Tudors: Jonathan Rhys Meyers does a fairly good British accent through seasons one and two. It began slipping a little more obviously in season three, then in season four, it appeared that he'd all but given up. Who knew that Henry VIII was actually Irish?
The Umbrella Academy (2019): Tom Hooper's American accent as Luther is generally good, but his natural English accent slips out occasionally, with Five being the biggest one.
Robert Sheehan's native Irish accent makes an appearance on occasion, such as the line "This is my nicest outfit!"
Waking the Dead: Felicite de Jeu may have done this once. She could have been putting on a more French accent than her real one.
The West Wing: There is the character of Lord John Marbury, played by Welsh Roger Rees, who speaks with an appallingly bad English upper-class accent. This is lampshaded at one point with Leo commenting that he doesn't think the accent is real.
The White Queen: Aneurin Barnard's Welsh brogue can be heard when his character Richard of Gloucester (who is English) utters, "How can he know the dog was poisoned? The thing was old, it could've died from anything."
This happens quite often any time a player is required to fake an accent. Other players would often take notice and point it out, lightly disrupting the scene. In some cases, a player's accent may shift multiple times in a single game.
The most memorable example of this was in a game of "Hollywood Director" where the performers were all playing Spanish characters. Ryan forewarned that his Spaniard "had a bit of Italian in him", Kathy's Spanish maiden was more French than anything else, and Wayne's Zorro was a stereotypical Mexican. "Funny how we all come from a different part of Spain!" observed Ryan.
Ryan had another moment of hilarity regarding his accent while playing a game of Whose Line. Despite playing Louis XVI, Ryan spoke mostly in an Italian accent, which he quickly justified by saying that while they were on vacation in Italy, he picked up the language. Once the game was over, he proved he could have spoken in a French accent if he wanted to, but claimed: "it was more fun in Italian."
In the same skit Colin lampshaded his slippage beforehand with "I'm so panicky that I'm sure my accent will go all over the place."
You'd think that with all the times Ryan has been told to do a specific accent but used Italian instead, telling him to do an Italian accent would mean he could finally do it correctly. Wrong. Danish.
It was lampshaded again in a game of Scenes from a Hat: Celebrity Endorsements Doomed to Fail.
Wayne: The Whose Line guide to accents.
The original British version occasionally called for fake American accents (Paul Merton in particular just sounding like a Londoner in dire pain) and, at least once, for Ryan and Colin to do "Shakespeare," which was cruel.
Also in the British version, Colin did such a spectacularly awful Scottish accent that he wound up referencing it in the American version. Hilariously, Colin was born in Scotland and as a boy took pains to hide his accent.
For those who don't know why it's "cruel", Ryan is American and Colin is Scottish-Canadian.
Dominic West (born in Yorkshire, England) is the very much Baltimore-born Jimmy McNulty. McNulty is supposed to have a stylized Baltimore accent, but he lapses into his British accent in the second-last episode of the first season during a conversation with a district attorney. Stranger still, at one point during the show's second season, McNulty attempts to speak like a British man to impress his fellow detectives, but ends up speaking in a highly exaggerated tone that is nothing like his original accent. Near the end of episode 3.4, as well, West blatantly slips when asking "You ever been down the ocean?"
Idris Elba's accent slips a few times, most noticeably in the very dramatic scene where Stringer admits to Avon that he murdered Avon's nephew.
Look for this in Idris Elba's run on The Office, too. Charles Miner sometimes becomes distinctly British (especially when angry).
Irish-born Aiden Gillen (Tommy Carcetti) mostly stays in accent except for one scene where, in playing Battleship with his daughter, he calls out "Haitch-six".
Wishbone: In episodes where the literary figure Wishbone plays is English, Wishbone's voice actor, Larry Brantley, attempts a bad English accent which he tends to keep slipping out of.
Without a Trace: Anthony LaPaglia and Poppy Montgomery are guilty. Within the realm of possibility that Marianne Jean-Baptiste has also.
Wonder Woman: The third season has a leprechaun. The Irish accent is limited to the actor saying "me gold" every couple of minutes. "Never talk about me gold!"
Lucy Lawless (another Kiwi) suffered serious vowel slippage in what was, presumably, supposed to be a Fake American; in all fairness to Lawless, though, that was hardly the greatest of the series' anachronisms.
Listen to her say "out of the ordinary" Season 1 Episode 22, around minute 18.
Canon has it that Mulder was born in Massachusetts, but David Duchovny plays him without any real accent. Which made it even more amusing to have heard Agent Mulder once lapse into Duchovny's own New Yorkese when he was angrily calling someone "stoopid".
Ditto Gillian Anderson. She was born in the U.S. but lived in England until she was in her early teens, and didn't try to shed the accent until she was out of high school. In the very early episodes, she'll slip occasionally. In the 2008 movie, it can get quite noticeable in Scully's emotional scenes, as Anderson moved back to England after the show ended in 2002, and had to re-learn her American accent all over again for the film.
The CSM is a government operative from somewhere in the US. One episode speculated that he was born in Baton Rouge, Louisiana, although this was never confirmed. Diphthongs, though, frequently show up William B. Davis' Canadian background.
21 Jump Street: Has a glorious example: a Polish exchange student speaks with what sounds like a badly done French accent. Since French and Polish accents sound nothing alike, the result is not so much a slipping accent but an accent that is already crumpled around the actress's ankles and forces her to scuffle through her part.
In a particularly bizarre example, in one episode, Sydney and Vaughn are portraying a French couple with requisite over-the-top faux accents—which is odd when one realizes that Michael Vartan is French-born. However, in the same scene, Vaughan speaks to their mark in perfect French, no trace of the false accent.
Even more impressively, though, Vartan in those scenes when he speaks English with a French accent sounds exactly like a French person speaking English with an accent would. (Take it from someone with a degree in French and who has a foreign exchange student from France one summer!)
Jennifer Garner, on the other hand, fakes multiple accents over the course of the show and does a terrible job of it. This is forgivable when Sydney isn't trying to pass as a native speaker of the language in question, but when she is, it's flat-out cringeworthy - her Russian and Japanese accents in particular.
In season 4, the English vampire Spike twice tries to fake an American accent. In this case, however, actor James Marsters wasn't using his own accent - just doing a good imitation of a poor imitation of one.
Anthony Head uses his actual British accent instead of his normal fake British accent as the candy-influenced teenage minded Giles in Band Candy.
The hero's ex-girlfriend Fiona is supposed to be Irish. After making the audience suffer through Gabrielle Anwar's extremely poor job at faking an Irish accent in the pilot episode, the creative staff decided to face up to the reality that she simply couldn't pull it off. Voila, episode 2 of the show has Fiona announcing that since she was living in America, she was going to work on 'faking' an American accent, and the horrible faux-Irish speech went over the side.
She's still faking it. Gabrielle Anwar is English.
When Michael went undercover as an American arms dealer while also playing an Irishman he almost sounded like he was about to slip back into the Irish accent a few times, despite the actor being American.
The actor who plays Michael, Jeffrey Donovan, is from Massachusetts although the character he plays is from Miami. He seems to really enjoy getting the chance to play up his natural accent when a cover ID needs to be from Boston or another part of New England.
Yvonne Strahovski uses her natural Australian accent when impersonating a scientist in one brief scene. She also spoke Polish (which is her actual first language) for a single line in an earlier episode. She's said the American accent gives her the most trouble on the word "girlfriend," but she slips very rarely.
Her accent slips noticeably on the word "orange." Unfortunately, her character Sarah Walker works in a frozen yogurt shop called Orange Orange.
Dark Angel: Bronx-born Alimi Ballard played his Caribbean immigrant character Herbal Thought with an accent so thick fans complained about not being able to understand his dialogue. For the second season of the series, Herbal began speaking something closer to standard English, claiming that his wife was making him do it so he could get a better job. After Dark Angel was canceled, Ballard played sharp, well-spoken, highly literate FBI agent David Sinclair on NUMB3RS. Apparently, the advice of Herbal's wife paid off for him!
In "New Earth", Lady Cassandra takes over Rose's body, and so Billie Piper speaks with her native Received Pronunciation accent rather than the Cockney accent she normally uses as Rose. Then she puts on a really bad Cockney accent for the Doctor, until he figures out it's her.
In "Tooth and Claw", the Tenth Doctor affects a Scottish accent in order to blend in as a native of the Scottish highlands. Later in the episode, he forgets to continue using the accent and is caught by Queen Victoria. Notable for the fact that David Tennant is, in fact, Scottish, and normally affects an Estuary English accent when playing the role. And for the fact that the Scottish accent the Doctor affects isn't Tennant's regular one.
There was a very specific reason that the producers didn't use Tennant's regular accent, which was that Eccleston had used his regular accent, specifically a north England accent which turned into an in-series joke. ("Lots of planets have a North!!!) The original intent was to use Tennant's normal accent, but the producers wanted to avoid turning Doctor Who "into a linguistic tour of the U.K.".
Of course, the Doctor's had a Scottish accent before Seventh Doctor Sylvester McCoy used his normal speaking voice in the role.
And after the Twelfth Doctor is played by the equally Scottish Peter Capaldi.
The 8th Doctor as played by Paul McGann speaks in a much more generic English accent than McGann himself, whose true voice has a distinct Liverpool lilt.
Gracepoint: At one point David Tennant raises his voice, and starts sounding oh-so-very-Scottish all of a sudden. The show takes place in California, and his character is supposed to be an American.
Life on Mars (2008): One episode of the US version sees Irish actor Jason O'Mara (who is playing New York-born Sam Tyler) going undercover as an Irishman, allowing O'Mara to use his own accent. Of course, there's the inevitable scene where his "Irish" accent is shown to slip-up and become "American" again.
The Nanny: In one episode, Fran Drescher's character loses her trademark nasal voice after eating a lot of wasabi, allowing the audience to briefly hear Drescher's voice without the affectation, though truthfully she's making an effort to sound even deeper than her actual speaking voice.
The Riches: In one episode, Wayne and Dahlia, played by Eddie Izzard and Minnie Driver, affect British accents for one part of a scam. Both Izzard and Driver are British and use fake American Accents (in her case, a rather thick Hillbilly drawl).
Six Feet Under: In one episode, Nate has a hallucination that Brenda is speaking undecipherable Australian slang in an Australian accent — which is actress Rachel Griffiths' real accent.
Undercovers: Nearly every time Samantha Bloom has to go undercover as someone other than an American, especially when she's blatantly turning on the sex appeal, Gugu Mbatha-Raw breaks out her original English accent.
Diana (Marsha Thomason) goes undercover as a journalist's personal assistant and puts on her natural British accent to play up her qualifications, as the journalist studied in England and would be more likely to hire someone from there.
Neal (Matt Bomer) is one of the men featured in a bachelors auction in Veiled Threat, and brings out his native Texan twang for the episode.
Who's the Boss?: Alyssa Milano is an interesting case. She was born and raised in Brooklyn, and so had the aforementioned accent, but she worked diligently to get rid of it in order to have a successful theater career. As a result of this, when she landed her big role on this show, she found it somewhat difficult to get the Brooklyn accent back. On the show, it's very inconsistent until they dropped it altogether.
Wynonna Earp: In the episode "Steel Bars and Stone Walls," Waverly sneaks into a secret headquarters by pretending to be from Scotland Yard. Dominique Provost-Chalkley uses an exaggeratedly posh version of her native English accent.
Examples in which this trope is in-character:
24: Everyone associated with Dana Walsh's past has a typical Deep South accent, but she usually doesn't. When she's especially rattled, however, her original accent slips out. (Her actress is originally from Oregon.)
30 Rock: Liz tries to pull off a Jamaican accent on the phone with Kenneth. She begins to call the other person "me lad", Kenneth whispers that she's slipping into Irish and to cover she quickly proclaims, "Cool runnings, mon. Bobsled" and hangs up.
Avenue 5: In a first episode Meta Twist, Captain Ryan Clark, played by Hugh Laurie, speaks with a very House-like American accent. When disaster strikes the ship, Captain Clark starts nervously slipping into an English accent much like Laurie's own and eventually reveals he is British but the passengers find an American accent more reassuring.
Battlestar Galactica (2003): When Baltar is accused of making up his background of growing up a poor farm boy on what was considered the most rural and backward colony, James Callis switches his cultured English accent for a rougher, more rural one (Yorkshire, in fact) in telling his story about leaving home and learning to speak in a more upscale manner.
Bones: Intern Arastoo Vaziri's Middle Eastern accent is faked, and slips completely when he gets irritated at Dr. Saroyan. He was faking being "just-off-the-boat" so he fellow lab workers wouldn't make fun of his genuine Muslim religious beliefs. Bones knew something was off, as his accent was from the wrong country. She can tell these things.
Castle: Similarly, a money-making guru named Johnny Vong fakes a just-off-the-boat Chinese accent to make his fake Rags to Riches story that much more plausible.
Doctor Who: David Tennant's use of a Scottish accent similar to, but not actually, his real one in "Tooth and Claw" has been detailed above, but mention must be made of Rose's atrocious attempt at a Scottish accent in the same episode, which was done because Billie Piper just couldn't do a Scottish accent.
The Doctor: Oh, I'm—I'm dazed and confused. I've been chasing this...this wee naked child over hill and over dale. Ain't that right, ya... tim'rous beastie? Rose: Uh, uh ... och, aye! I've been...oot and aboot! The Doctor:[under his breath, to Rose] No, don't do that. Rose: Hoots, mon! The Doctor: No, really, don't. Really.
One episode had Ross inexplicably starting to teach a new class in a horrible British accent. His attempts to "gradually phase it out" as he "adapted" to America made it even worse.
Similarly, in the episode "The One with Ross's Tan", Jennifer Coolidge's 'Amanda Bufamontisi" is an American who has lived in England for long enough to convince herself she has an English Accent. It epitomizes Bad English Accent humor. So much so that Lisa Kudrow could barely make it through filming without laughing hysterically...to the point that in one take, she started laughing before Coolidge even said anything because she was thinking about how funny it was going to be.
GraceLand: Mike is going undercover and tries to incorporate an accent into his persona. Briggs tells him to knock it off since his accent is terrible and it will automatically make their targets suspicious. Unless Mike can do the accent flawlessly, he should just use his normal accent.
Highlander: The Series: This franchise has just about the best use of this trope ever. Given that most of the main characters are immortal and have been Walking the Earth for hundreds or even thousands of years, any lapses or shifts in accent can be tossed up as "I lived in *insert country* a while back". Adrian Paul plays up Duncan Macleod's Scottish Accent thicker depending on the time: the further back the flashback, the thicker the accent. Hell in one episode in modern times he hams up his accent to what it was when he was young in order to get another immortal to recognize him.
Jessie: The titular character is from Texas but doesn't have an accent except for a few occasions when she slips into one.
Kim's Convenience: Janet, a Canadian of Korean descent, fakes Asian Speekee Engrish and pretends to be a North Korean film-maker to a security guard in order to get into a film festival backstage. Unfortunately, she also runs into the same security guard a few more times in her family store and has to keep the accent/bad English up in each encounter. She slips into her fluent non-accented English a few times which confuses the security guard but her cover isn't blown until right at the end when she feels guilty and confesses.
Last of the Summer Wine: Edie puts on a ridiculous fake posh southern accent out of social-climbing ambitions, which has a tendency to vanish and be replaced by her natural salt of the earth Yorkshire accent at times of stress.
Lampshaded in the Late Night With Seth Meyers parody trailer Boston Accents when we learn that the cast includes "a British actor who's trying his best" and is of course shown to be just using the nasal dropped "r" in the context of his usual accent.
Law & Order: Criminal Intent: In the episode "Chinoiserie", Goren and Eames immediately identify a supposed British lord as a fake by his atrocious accent which keeps jumping around the UK. The impostor, an actor unwittingly roped into the con under the guise that it was performance art, insists that it is "a perfectly valid English musical hall accent".
One blurry example from The Office (US) is when Nellie Bertram is impersonating an American radio host as part of one of Jim's pranks toward Dwight. Catherine Tate's accent is damn near flawless, save for one line when she pronounces the word "consumer" the British way ("con-syoo-mer" as opposed to "con-soo-mer"). However, given that both Tate and Nellie are British, it could either be an actual slip-up by Tate or an in-character slip by Bertram to add a sense of realism (professionally-trained actor Catherine Tate might be able to pull off a flawless American accent, but salesperson Nellie Bertram might slip up here and there),
When the clones (who include an American, two Canadians, two Englishwomen, a German, and a Ukrainian) try to impersonate each other, they don't always get it right. In particular, Alison's impersonation of Sarah in the fourth episode. Actually pretty good acting on the part of Tatiana Maslany, who handles all the clone characters.
For characters that are supposed to be English, Sarah and Felix's accents are all over the place. Justified by having lived away from their home country for so long. Averted with Rachel, as her accent is consistent.
True Blood: There is one episode in season 2 where Eric Northman, a Norse vampire (yes, really) played by a Swedish actor with a neutral American accent, pretends to be a human with a Southern accent. The result is horrifyingly hilarious (and very obviously bad on purpose).
Xena: Warrior Princess; in the episode "The Xena Scrolls", the framing sequence is set in the 1930s with archeologist Janice Covington (Renee O'Conner) and linguist Melinda Pappas (Lucy Lawless) discovering Gabrielle's scrolls depicting her adventures with Xena. During the episode, they meet a French soldier named Jacques Suer (played by Ted Raimi), who sports a comically bad French accent. It is later revealed that Jacques is really "Jack", a New Jersey native who joined the French military after being declared 4-F during his Army physical. He promptly dropped the accent at that point.