Just a general notice, that blond Australian actresses (with the exceptions of Yvonne Strahovskinote What makes Strahovski's ability so impressive is that English isn't even her first language. She grew up speaking Polish, then learned to speak English perfectly (with an Australian accent), and then learned to speak English with a nearly flawless American accent. In a few episodes of Chuck, she has also convincingly spoken in a Southern accent and a British accent. and Toni Collette) seem to be incapable of accurate American accents. Sarah Wynter is the most egregious offender, followed closely by Poppy Montgomery and then Anna Torv. Portia de Rossi isn't too bad, but she slips up every so often. Strahovski does a perfect accent, but it's obvious by the way her mouth moves when she talks that she is speaking in a different manner than normal.
Examples in which this trope wasn't intended to happen:
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.
Alexis Denisof also slipped now and then on words like "data".
"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 noticably, or the use of inherently British words such as 'bollocks' a little too much like how an American would say them.) However, during season 4, twice, he has to fake a Southern accent ("I am a friend of Xander's") and the results are (perhaps intentionally) hilarious.
This gets brain bending when you realize that this is an American man (James Marsters) pretending to be an upper-class Englishman (Spike's actual origin) pretending to be a lower-class Englishman (Spike trying to sound tougher) pretending to be an American (to fool Riley).
To an actual Brit, on Spike's earliest appearances in season 2 his accent comes across as exaggerated, with frequent slips, becoming more natural in season 3 - making the jarring moments beyond that quite awkward. His British accent also comes across as more Australian at points.
The problem doesn't go away when he plays Capt. John Hart on Torchwood with an equally slipping British accent.
Kendra, played by Bianca Lawson. Supposedly the accent was a last-minute addition, and the dialect coach taught Lawson an accent from a very specific, obscure area of Jamaica. To both viewers and crew it just sounded like a lame Jamaican accent.
Are You Being Served?: Mrs. Slocombe tends to affect an awkward Received Pronunciation accent when talking to customers or otherwise trying to appear sophisticated, but lapses into her (and actress Mollie Sugden's) native Yorkshire accent when angry, embarrassed, drunk, or speaking more casually.
This is part of the character's pretensions rather than the actress not being able to hold her accent though.
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.
During the same trial, English actor Jamie Bamber, who plays Lee "Apollo" Adama with a pseudo-American accent, slips up and says, "Chamallar extract," adding in an "r" sound after "Chamalla," which is a British-ism.
And while Bamber did an admirable job on the whole in the series, his accent in the mini-series does slip on occasion, though this is partly forgivable since the series isn't set in the real world at all but in a made-up set of planets, and his accent was faked more for the purpose of matching Edward James Olmos's than in order to sound like an actual American.
Perhaps the best example of a slip came in 2008, when the cast appeared on David Letterman's show to do the Top Ten List. Bamber's line consisted of technobabble, which he recited in an American accent in the rapid-fire manner of Adama, before clearly dropping the accent as he suddenly declared "I don't know what the hell I'm talking about". Fridge Brilliance kicks in when you realize that he probably did that deliberately, seeing as how he was breaking character.
Bamber has similarly slipped up or struggled to maintain an American accent (despite an overall good job) during some of his appearances on other American TV shows—Cold Case (sure enough, during a "highly emotional scene"), CSI: Miami, and 17th Precinct (an unaired pilot). Very tellingly, the latter two came after several years of him using his normal accent while on Law & Order: UK, so one can assume he was simply out of practice, but in Precinct, he fumbles it so badly that the result is a bizarre hybrid of British, American, and Canadian (the episode was filmed in Vancouver)
Even his natural British accent isn't exempt from this, as he also had some trouble maintaining a working-class accent while playing a firefighter on The Smoke, as it was a notable difference from his natural upper-class one.
Tahmoh Penikett slips into his own Aboriginal/Inuit accent regularly in the first season and 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.
His "normal" accent is more complicated: his father, Tony, moved to Canada from Great Britain in 1957 when Tony was 12, and still noticeably has an English (Sussex) inflection when he speaks, which influenced his son.
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.
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.
John Ross Bowie plays Barry Kripke, a character with rhotacism, though at times slips into his normal speaking pattern.
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.
Brookside: There was a 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 traveller 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.)
Gives us 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?" Ahh, much better.
And Anwar is quite English, so her fake American is even better than that.
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.
Charmed: 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.
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 any sort of regional accent!
He deliberately changed his accent when quite young, and never slips back into his original Southern accent - except once (fandom consensus was that this was adorable).
He does always pronounce "egg" as "ayg."
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.
Cult: Stars Robert Knepper, and he's doing basically the same accent he used on Heroes.
Al Swearengen's accent waxes and wanes like the moon. Unlike the real historical figure, Ian McShane plays him as an immigrant and even mentions Manchester (nearby to McShane's native Lancashire) within the dialogue. This would explain slippage.
In contrast, Paula Malcomson who plays Trixie is originally from Northern Ireland but her accent hardly slips at all.
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.
Doctor Who 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 overpronounced RP at first, which soon relaxes to a much more neutral Southern accent. This was because the actress 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 favour 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.
Tom briefly goes Scouse during his Dropping the Bombshell line in "Nightmare on Eden": "I don't work for anyone... I'm just 'avin' foon."
In "The Horns of Nimon", there's a flub where he starts addressing K-9 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".
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.
John Barrowman, who plays Captain Jack Harkness, is British 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 on Family Guy).
Worth mentioning that Captain Jack isn't American; he's from the Boeshane Peninsula in the 51st Century.
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 for the episode "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.
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"
"Tooth and Claw" had a kind of in-universe example, since they were in Scotland and Ten was faking a Scottish accent (a little broader than Tennant's native one). When the Monster of the Week shows up and he drops back into the Doctor accent, the Queen calls him on it.
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 "Day of the Doctor", Matt Smith's Eleventh Doctor insults Tennant's 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.
Tennant also slips into a neutral posh English accent quite a lot during the specials, most notably "Waters of Mars". This is because Tennant had been playing Hamlet with the Royal Shakespeare Company for months, and 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 "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.
Paul McGann's Eighth Doctor has a subtle case of this. His accent travels a sort of spectrum between Received Pronounciation 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.
To be fair this is pretty much Paul's standard "acting" accent - he rarely uses his natural, noticeably Scouse voice in his roles, including his extensive voiceover/narration work.
He acknowledges it in the commentary; apparently he was too tired to stick to his "acting" accent.
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.
In "The Stolen Earth", Martha Jones's 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.
Sanchez is played by an American actor (Micheal Brandon), albeit one who has spent the past thirty-odd years living in the UK.
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.
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 speak English peppered with French 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.
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.
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 in 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.
One episode required Daphne to fake an American accent, complicated by the fact that the only word she could say in said accent was, "Sure!". This is especially notable considering that Daphne's own English accent is faked by actress Jane Leeves.
Her fake Mancunian accent is pretty unconvincing to British viewers. Jane Leeves is British but was born in Essex and raised in Sussex — nowhere near Manchester.
It is, however, completely subverted by John Mahoney, who was also born and lived in Lancashire into his teens. Unsurprisingly, he once pulled off pretty good Manchester accent when doing a deliberately mocking impression of Daphne.
Jane Leeves's weird and allegedly Mancunian accent was developed by the actress as a generic approximation of a North-Of-England accent - i.e. a British accent that would sound more palatable to American ears - despite it not being at all realistic or representative of any one geographical part of England at all.
Alternatively, her accent can be thought of as slipping and sliding along the North Cheshire Plain from Runcorn and Ellesmere Port right across to Stockport and back again... only at the Stockport end of the Grand Tour does it even touch on Manchester, and then on a South Manchester accent. Americans would not have liked North Manchester at all, as this touches on broad Lancashire.... tha' knows, sithee, yon Yanks'd've bin reyt up in't urr.
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) than 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 "haaay err 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.
Friends: Happens In-Universe in the episode "The One with Ross's Tan". When Monica and Phoebe's old friend returns from England, she fakes a British accent. At one point, she says "I feel like a perfect arse", pronouncing "arse" with a rhotic R.
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. Jack Gleeson as Prince Joffrey also occasionally slips into his native Irish brogue.
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.
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.
Hannibal: 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.
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 Laughlin usually does very well in maintaining his fake American accent to play Steve Mc Garrett 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).
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, including Rachael Taylor as Trish Walker, Eka Darville as Malcolm, and Wil Traval as Will Simpson. Of the three, Taylor's is the least convincing.
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.
Both Magnus Scheving (who plays Sportacus) and Stefan Steffenson (who plays Robbie) sometimes slip into their native Icelandic accents.
Sometimes? Sportacus doesn't even seem to be trying to hide his accent, though this is most likely because he hails from "an island in the North Sea."
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.
Dear Gina Bellman, we'll argue about whether or not you're an excellent actress or a poor one. In some way, your power is in that argument. But, the accent, oh, the accent. Whenever you take on a non-Brit accent, it weaves like a drunk.
Which might be part of the point, if you really think about it. Most of her marks are American businessmen. In one episode ("The Rashomon Job,") she mentions that to Americans, all accents sound the same (during the various flashbacks, the members of her team portray her accent as Cockney, Scottish, and pure indecipherable gibberish.) It might be possible that Sophie isn't trying to sound authentic in her cons, but is trying to sound the way her target expects her to sound.
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 of back in. This gets even stranger once you realize that the actor is from Texas.
There are numerous American movie or television shows in which a character has tried and failed to do an Australian accent, often to the point where she doesn't even realise it is meant to be Australian until someone else points it out. Notable for its bad Australian accents is this show, with over five different accents being called "Australian". Only Victoria native Emilie de Ravin does a genuine Aussie accent (and even then, some viewers complained about how "fake" her accent sounded).
The worst example of a failed Australian accent is Claire's mother, Carole, who is portrayed by a British actress. Especially for Australians, her scenes are very hard to watch.
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. When his accent slipped in "There's No Place Like Home," many Americans didn't notice, but Brits and New Zealanders did.
It slipped earlier than that too, towards the end of "The Shape of Things to Come", prompting some on Lostpedia to wonder aloud if there was an actual, plot relevant reason for it.
Sun originally spoke with a Korean accent for most of the first two seasons of the show. However, at some point in the series the accent completely disappeared and Sun inexplicably spoke with a perfect American accent.
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 on 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.
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 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.
Corky Sherwood would slip into a southern drawl whenever she became extremely angry.
Corky (like Faith Ford) is from Louisiana. As a news anchor, she'd have had to learn to drop the accent on-air, so it makes perfect sense for her to revert when stressed.
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.
The Night Shift: Eoin Macken's Irish accent tends to pop up whenever TC is angry/ emotional/ yelling.
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, the actors were predominantly from New Zealand and Australia, and the accents were invariably transient. Until Haim Saban bought back the franchise, the last season in which the Ranger actors were primarily from North American was Power Rangers S.P.D., though Monica May (Z) was the only American; the other actors were Canadian.
Brancatisano and Firass Dirani, who played Nick, had actually initially auditioned for each others' roles (Brancatisano had auditioned to play Nick, and Dirani had auditioned to play Xander). While Dirani's American accent proved to be fairly decent, Brancatisano's American accent was much less so, as shown in this commercial for the Cosmopolitan Hotel in Las Vegas; he's the young man who uses an unconventional method to try to chat up a young woman at the bar, only to be confronted by the woman's (presumed) bodyguards.
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).
This seems to be especially noticeable whenever there's a kid actor on the show: In particular, in Power Rangers Samurai, whenever Jayden and Antonio appear in flashbacks, the kid actors simply cannot hide their accents. Particularly notable in that present-day Antonio is played by Steven Skyler, an American of Thai and German descent who usually succeeds in affecting a Hispanic accent, though even he occasionally slips up from time to time.
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.)
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.
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.
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.
At the start, Deanna Troi spoke with a pronounced Eastern European accent that softened over time to an mid-Atlantic accent somewhat closer to her natural English accent. Her mother, Lwaxana Troi, has no such accent. After Marina Sirtis called the producers on this, they decided she must have been using her father's accent. Then, in a later episode, an illusion of her father shows up sounding nothing like her either.
Or else actress Marina Sirtis, a Londoner born to Greek Cypriot parents, was recoursing to a very specific Eastern European accent - as Greece and Cyprus are both at the eastern end of the continent and all she would have needed do is speak English as her wider Greek family spoke it.
Explained (or lampshaded) in a later episode where Lwaxana laments letting Deanna spend so much time with her nanny as a child, because the nanny's accent rubbed off on Deanna.
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.
Of course Scotty's accent isn't exactly what one might consider 100% authentic anyway.
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.
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.
However, the actually British would know that although they say chips more often than fries, saying fries would be perfectly acceptable, making this clue useless.
Hilariously he is played by real Brit Charles Shaughnessy.
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".
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 everyone, 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 (Nikki) sometimes has trouble keeping her native accent from coming through as Nikki.
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.)
Top Of The Lake: 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.
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.
Though one actress manages to slip into two different Southern accents and random points.
The show also has in In-Universe example with Russell Edgington, who loses his heavy Southern Accent in favor of a German sounding one when he gets angry. Justified since while he is older than both Germany and the New World he has been a Southerner for a relatively short time, and was a Nazi before that.
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. I never knew that Henry VIII was actually Irish.
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.
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.
Actor Dominic West (born in Yorkshire, England) played a Baltimore detective in the HBO series. He 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, his character (Jimmy 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?"
And fellow Brit Idris Elba's accent (as Stringer Bell) 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 Minor sometimes becomes distinctly British (especially when angry).
Irish Wire actor Aiden Gillen (Carcetti) mostly stays in accent except for one scene where, in playing Battleship with his daughter, he calls out "Haitch-six".
Wishbone: 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.
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 scene 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.
Buffy the Vampire Slayer: 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.
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 a 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. Dark Angel was canceled, but Ballard now plays sharp, well-spoken, highly literate FBI agent David Sinclair on NUMB3RS. Apparently the advice of Herbal's wife paid off for him!
In the episode "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, the actor playing the Doctor in the episode, 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 tour of the U.K.".
Of course, the Doctor's had a Scottish accent before - 7th Doctor Sylvester McCoy used his normal speaking voice in the role.
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.
In "The Faceless Ones", the alien duplicate of Jamie has Frazer Hines' real-life English accent.
Life on Mars (2008): One episode of the US version sees Irish actor Jason O'Mara (who is playing New York born Sam Tyler) go 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.
In one episode, Fran Drescher's character loses her trademark nasal voice after eating a lot of wasabi. Though the effect is only temporary, it allows the audience to hear her real voice.
It's not quite her real voice - Fran Drescher literally has to talk a lot slower to sound less Fran-ish.
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.) Then you remember that they're playing Irish Travellers, and you get even more confused.
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.
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.
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.
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.)
Phoebe's British accent. Her actress, Emily Mortimer, actually is British, making her a Brit-playing-an-American-playing-a-Brit.
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.
Battlestar Galactica (2003): When Baltar is accused of making up his background of growing up a poor farmboy 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 his own Scottish burr in "Tooth And Claw" has been detailed above, with The Doctor "pretending" to speak with a Scottish Accent; but Rose's attempt also bears mention here:
Doctor: I've been chasing this wee child over hill and dale. Ain't that right, you...timorous beastie?
Rose: ...Och, aye, I've been oot and aboot!
Doctor (under his breath, to Rose): No, don't do that.
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 epitomises Bad English Accent humour. 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.
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.
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.
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.
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 outraged impostor, an actor unknowingly hired to play the part as part of a con, keeps insisting that it is "a perfectly valid British musical hall accent".
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.
Roswell: Emilie de Ravin ends up letting her Australian accent slip through in a couple of places in her first appearance on the show.
True Blood: There is one episode where Eric Northman, a Norse vampire (yes, really) played by a Swede 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).
The X-Files: Agent Mulder is from Southern New England — but sometimes David Duchovny's New York roots show through.
Grey's Anatomy: As per the general notice at the top, 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.
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 delcared 4-F during his Army physical. He promptly dropped the accent at that point.