In Invictus, Morgan Freeman makes an attempt to sound like someone from South Africa but slips back into his natural accent halfway through each line.
In Lara Croft: Tomb Raider, Daniel Craig plays Lara's overtly American rival and slight love interest Alex West. For the most part, he does a good job maintaining the accent except for one moment in the temple with the first piece of the film's Macguffin. As he climbs up a ledge you can clearly hear him say in the most British accent possible "Oh blast!"
In August Rush the English actor Freddie Highmore plays an American boy who's lived just outside New York all his life. Granted it was the first time Freddie had to put on an American accent, but it was weak and often slipped.
In The Town the English actress Rebecca Hall plays American bank manager Claire Keesey. Her accent subtly slips throughout the movie, one example being in the scene where her character and Doug (Ben Affleck) are on a date and run into Jem (Jeremy Renner). The line "So I've been telling all my friends about you," is clearly said in a British accent.
In The Third Man, Alida Valli (playing Anna) obviously believed that it would be appropriate to use several English accents in her role as an east-European escapee of Communism; One time it's Russian, another time it's perfect British English, then it's German English, then one time it's American English, then Dutch English.
In Stardust, Michelle Pfeiffer plays an English witch with English sisters, who have actual English accents. Pfeiffer attempts an English accent, which works now and then in the film, but often sounds horrendous.
The King of Accent Slippage is Sean Bean. His native South Yorkshire accent escapes in virtually all of his movies — including GoldenEye where he refers to James in a way that wouldn't be out of place in Emmerdale. Count Vronski in Anna Karenina wasn't from Sheffield.
Liam Neeson, usually when he has to say anything with an "oo" sound, like "you". He's become better at it as he's gotten older, but some of his early films like Darkman are a fairly egregious case of this trope.
Brit Claire Forlani plays Brandi Svenning in Mallrats, set in New Jersey. Her accent is decidedly unJoisey and her natural English accents slips in quite often. It's especially noticeable in the opening scene, filmed long after the original production wrapped and absolutely no one was happy to be there, according to director Kevin Smith.
In The Dark KnightGary Oldman's accent slips throughout the movie but is especially evident during the scene on the roof when Gordon, Harvey Dent, and Batman are discussing what to do about Lau.
Nim's Island: Butler plays Alex Rover with his natural accent, as well as Nim's father, with a Fake American accent that is truly embarrassing to hear, even when the Scottish vowels and Rs aren't forcing their way out. Best seen in the scene where Nim and her father are reunited. About a minute of relieved, Scottish-accented ranting, then he switches back to American in the space of a glance.
In general, Ray Milland had a hard time hiding his Welsh accent. Specifically in Easy Living, his American accent comes and goes, mixing into a strange hybrid at times.
In the Australian slasher film Nightmares from 1980, the central character's accent swings back and forth wildly between something resembling an Australian accent and something resembling an American accent. She was an American trying to sound Aussie, but her accent lapsed so regularly that it really was a 50/50 chance guessing which was the real one.
In the Conan the Barbarian (1982) film, surfing champion Gerry Lopez played Subotai the Hyrkanian with a vaguely foreign accent (the Hyrkanians were effectively proto-Mongols), but occasionally slips into a California accent.
Though not a fictional role, per se, in Christian Bale's Epic Terminator 4 Rant, his normally convincing American accent slips generally in a direct relationship to his frustration and rage.
Although Bale's accent is his own bizarre Transatlantic creation nowadays. His Welsh birthplace is often mentioned but he has never sounded remotely Welsh - probably because his English family only lived there temporarily. His natural voice (seen in his earliest movies like Empire of the Sun) is RP Southern English.
In the actual film, Sam Worthington's American is extremely shaky (it still wobbles a bit in Avatar, but he has improved). Worthington reverts to his own accent for several minutes in the middle of the film, when he's in bondage and having a heart-to-heart with Bale, making one wonder how it was able to slip past the cast, the director, the crew, the editors, and the test audience.
His American accent in American Psycho, while at times jarring, actually works there, with its imperfections actually helping to subtly enhance the nature of Patrick Bateman's character. While the dialect is usually spot-on, it comes off as obviously put on, as if Batemen is trying way too hard to sound normal. One noticeable slip does occur though in the limo scene near the beginning of the film, with his mangled pronunciation of the name Palmer.
Bale's accent slips a few times in The Machinist, especially when he's angry, but in other places too. Check out the scene at the DMV, particularly the line "I wouldn't ask if it weren't extremely important."
In The Dark Knight, in the scene where Bruce Wayne crashes into the police escort, Bale's accent slips away.
In Public Enemies, Bale's portrayal of South Carolina-native Melvin Purvis has him attempting a southern drawl every other scene.
The 39 Steps has Robert Donat playing a Canadian vacationing in London. Somewhere around the final act, he slips more and more into his British accent.
Viggo Mortensen's mish-mash of a natural accent makes Aragorn's accent roam wildly around the globe when it slips: variously RP English, American, or lilting Welsh, and at one or two moments he even sounds like a Newfie. Interesingly, he speaks Elvish with his own accent.
Sean Astin slips just once, in Rivendell, when he says, "We did what Gandalf wanted, didn't we?" The 'wanted' comes out sounding American.
There are a couple of times when Elijah Wood's accent seemed to be wavering, though it is mostly good.
In The Insider, the British Michael Gambon has one scene as Brown & Williamson CEO Thomas Sandefur in the Louisville, Kentucky scenes. His accent goes back and forth between the South and England with the line "It's spooky how he can concentrate!" Otherwise, he sounds very much southern just like the real Thomas Sandefur was.
In the film Mary Poppins, Dick Van Dyke puts on a painful cockney accent that comes and goes depending on what scene he's in. Averted when he plays Mr. Dawes Sr, who has a pretty dead-on English RP accent.
In Blood Diamond, Leonardo DiCaprio does a passable Rhodesian accent until one scene in which he shouts at Djimon Honsou for going another way. As he raises his voice, he reverts back into his normal American accent.
Kate Beckinsale as Anna Valarious in Van Helsing is unable to maintain an Eastern European accent for longer than a couple of lines at a time. She eventually gives up completely about halfway through the movie. To their credit, the Brides of Dracula didn't even try to sound anything other than American (even though only one of the three brides is actually American).
In Volunteers, Tom Hanks' cartoonish New England "Haahvahd" accent flips on and off like a lightning bug.
He uses a more realistic New England accent in Catch Me If You Can. It's improved a lot in 20 years but still wobbles a little on occasion. In the outtakes, he attempts to say a line through a mouthful of food ("She thinks he's Connors") and then asks "but could you hear the accent?"
In Forrest Gump, the way he yells "Greenbow, ALABAMA!" is much more Tom Hanks' voice than Forrest Gump's.
Owen's American accent for Derailed slips for just a teensy line near the end where he pops in the hotel room to tell a stranger he's being scammed. The word "scam" comes out in his original British accent.
He seems to attempt an American accent for about half of the first scene in Inside Man. Drops it pretty quickly after that, and it never resurfaces.
Watchmen. As Adrian Veidt/Ozymandias, Matthew Goode deliberately slips into his character's native German accent when excited, but he also unintentionally slips into his own native British accent on occasion.
The Sci-Fi original movie Rock Monster has a supposedly Scandinavian character, who occasionally attempts a generic Eastern European accent.
Newsies: Christian Bale's Jack, in particular, sounds like he couldn't decide what New York accent to do and decided on all of them.
In Hannibal, Julianne Moore has Clarice's accent for her first few scenes and then drops it for the rest of the movie. Of course, Silence of the Lambs did establish Jodie Foster's Clarice was trying to lose her Southern accent, but who knew she'd accomplish it so suddenly? It's quite glaring in certain scenes.
"Yew swear tew!"
There's a scene in Madhouse where temperamental horror icon Paul Toombes (Vincent Price) has just been disrespectful of his female costar's alleged unprofessionalism. She draws herself up to her full height and tells him exactly what she thinks. And then...
Jackman slips into Australian in the first movie the first time he says, "Storm" (just before "What do they call you? 'Wheels'?"). And also in the truck, right after Rogue tells him that he should buckle up. And again when he says to Jean "Couldn't wait to get my shirt off again, could you?". And in the second movie, on the word "art" when talking to Bobby's parents.
In the first film, Anna Paquin at least attempted to remember to give Rogue a southern accent on occasion. Seems like she gave up by the end of the third film, though.
Also in the first film, Halle Berry speaks, very briefly, in a vaguely foreign accent. This is made even more evident if you watch some of the deleted scenes. She completely abandons this by the end of the film. She may have been angling for a Sub-Saharan African accent, what with Storm being from Kenya in the comics. But it's worth noting even director Bryan Singer referred to it as an "attempt" in his DVD Commentary.
As in the previous film, James McAvoy loses his English accent in favor of his native Scottish one when he shouts, "I don't want YA FUTCHA!" It's even more apparent when he's yelling at Erik on the plane because it comes out as, "YA ABONDONED MEH!"
Interestingly, it's averted with Michael Fassbender who maintains an Ian McKellen-esque English accent rather than slipping into his own Irish one as he did repeatedly in First Class.
The English Nicholas Hoult's American accent is pretty good, but he frequently messes up on the word "professor."
Hugh Jackman's Australian accent slips out in Wolverine's very first line of dialogue.
In Spider-Man 2, Alfred Molina, who plays Dr. Octopus, is from London. When he says, "I should've known Osborn wouldn't have the spine to finish you!", his British accent is audible.
Chocolat when Alfred Molina is talking to Father Henri doing his gardening. Molina forgets he is meant to French for the entire scene and speaks in his natural London accent.
In Thirteen Days, which is set during the Cuban Missile Crisis, Kevin Costner's Boston accent (he plays Kenny O'Donnell, a presidential aide to John F. Kennedy) is particularly atrocious. The other actors do better, but he horribly stretches out his vowels and just generally exaggerates everything in a really distracting way. It's especially noticeable considering that his character has a lot of the more dramatic dialogue, the other accented characters speaking a lot of stuff that is more-or-less lifted straight from transcripts of White House recordings.
Robin Hood: Prince of Thieves: Kevin Costner barely even attempts a British accent while playing the eponymous Robin Hood. Apparently, the accent he tried for the part was so horrendous that the director told him to speak normally.
In 21, English actor Jim Sturgess does what could charitably be called a decent American accent. Mostly. When he remembers.
In Goal!, Anna Friel, playing the Newcastle United club physician, does what could charitably be called a decent Geordie accent. Mostly. When she remembers.
The actors of The Shipping News attempted the Newfoundland accent with varying degrees of success. Most distracting, however, was American actress Julianne Moore's version of it, which usually settled on sounding like a badly-faked Irish accent.
Brendan Gleeson's American accent in Green Zone seemed to slip into Irish on the odd word which oddly enough made him sound somewhat Chicagoan.
About twenty minutes into A New Hope, Princess Leia inexplicably gains a clipped, pseudo-British accent when dealing with her Imperial captors. It disappears after she is rescued and never returns again. Carrie Fisher said that the British-accented scene was the first one she filmed, and the decision was made to drop the accent afterward, but there was no budget to re-dub the scene.
In The Empire Strikes Back, the snowspeeder co-pilot Dak slips into a British accent when he says "Feeling alright, sir?" to Luke.
In Attack of the Clones, Jack Thompson's Australian accent noticeably slips through every now and then, while he otherwise mostly speaks in an American accent portraying Cliegg Lars.
Natalie Portman in V for Vendetta. Evey Hammond lives in London. Portman's accent in the movie varies between Cockney, Australian, RP English, and a strained-sounding American. Strangely, it comes across as quite endearing.
Moulin Rouge!: most noticeably when he says "No matter how things get or whatever happens, it will mean that we love one another." to Satine. And since he's Scottish, every once in a while, his "oh" sounds will sound like "oo." (Like "humble abood" and "goo away.")
He has an American accent in The Men Who Stare at Goats, but on certain lines ("You forgot your caaaaahp!") it lapses noticeably. It's pretty noticeable when he says "No, I won't" during the Elephant Love Medley Scene.
In Doom, Karl Urban holds up his American accent awfully well. But as the film goes on, it gets shaky, especially when his character starts to yell.
Practically the entire cast of Doom, all of whom are supposed to be Americans, are British. This may be down to the fact that it was shot in eastern Europe, where work permits are easier to get for EU locals. Even Richard Brake, playing Portman, was actually born in the UK (in Wales), though to all intents and purposes he's American. The only American-born American in the entire cast is Dwayne "The Rock" Johnson. Rosamund Pike (Samantha) is famously English, as are Deobia Oparei (Destroyer), Ben Daniels (Goat), Raz Adoti (Duke), Al Weaver (The Kid), and Yao Chin (Mac - going by the family name Takahashi, his character is supposed to be of Japanese descent, but somehow has three names, which is unknown in Japan). Dexter Fletcher is also in it with a risible attempt at American, but the others seem to do pretty well.
In The A-Team, Sharlto Copley uses a vaguely Southern accent which he has a hard time keeping up when he's excited (see the line "You gorgeous old rust bucket, you! Did you miss your daddy?", which is entirely in his natural South African accent) or shouting. Though given that it's Murdock, it's entirely possible a lot of it was intentional, particularly the 'gorgeous old rust bucket' and 'heat seekers' lines.
In fact, it's probably all intentional, given that in the original TV show, Murdock uses no less than five different accents in the pilot episode alone.
It's used deliberately in a brief scene where he impersonates a South African reporter.
Mel Gibson in Braveheart. ([Gibson's American accent] "We won at Stirling!" [Scottish accent] "And still you quibble!")
Gibson had a hard time ditching his Australian accent earlier in his career. The first two Lethal Weapon movies have lots of little bits and pieces of it peppered in. The "gold pen" speech in Lethal Weapon 2 stands out in particular.
Brit Aaron Johnson does a decent job with an American accent in Kick-Ass, although towards the end he mentions being troubled by the "idear" of never seeing his friends and family again when he thinks he's about to be killed.
In L.A. Confidential, it seems more like James Cromwell is occasionally slipping out of his native American accent. He plays the Irish-American Capt. Dudley Smith in the movie, but most of the time he sounds American, occasionally with a terrible Irish accent. It's most notable when he says Irish words like "boyo."
'Boyo', of course, being a stereotypically Welsh thing to say.
In the novel, it's made a bit more clear that Dudley Smith takes care to cultivate the stereotypical Irish Cop image so that people will underestimate him, in a sort of native variety of Funny Foreigner.
Kate Winslet managed a pretty decent American accent in Titanic (1997), but it did falter in a couple of places. Watch the sequence where she tells Jack that she is engaged. Although, considering Rose is returning from England, it could be argued that her character may have picked up an affected accent on the trip.
Winslet does far better when she plays the role of Clementine in Eternal Sunshine of the Spotless Mind. There are a handful of small mistakes, but they're not noticeable unless you're deliberately paying attention to her pronunciation.
In National Treasure, German native Diane Kruger's attempts at a nondescript American accent fall short quite often under scrutiny. The accent-masking is convincing when it isn't slipping, but the slips are very noticeable.
Justified as it's mentioned her character is German-born. Kruger can speak fluent English without an accent, so her slight German accent poking through is likely intentional. She even swears in German at one point.
Cary Elwes's agent (or manager, or somebody) apparently convinced him a few years ago that he should lose his native accent - he mentioned this on The Tonight Show or somesuch once. This falls somewhere below Ishtar, but not too far below, in Disasters which might have been avoided.
Brad Pitt is something of a serial offender in this field, most notably in The Devil's Own where he plays an IRA bomber hiding out in New York: he's full-on "terty-tree-and-a-turd" Oirish for the most part, despite his character being from Northern Ireland, but starts to lose it when the waterworks get going for the climax.
It appears he's not even trying in 12 Years a Slave. For a "Canadian" he sounds awfully Southern.
A Man Of No Importance: for whatever reason, Albert Finney is flat-out incapable of pronouncing the "R" sound. This makes it a little harder to enjoy his portrayal as an Irish theatre director trying to organize rehoouusals.
Marlon Brando in The Missouri Breaks. He starts off with an exaggerated ''top o' the mornin' " Irish accent, which is quite jarring since the film's set in the Old West and his character's named Robert E. Lee Clayton. But later in the film, this accent's gone and Brando's back to his natural voice. Supposedly Brando claimed that this was deliberate, to show that Clayton was mentally ill.
Burt Lancaster, an American, portrayed Irish born Colonel Anthony Durnford in the film Zulu Dawn. Lancaster's Irish accent slips in and out several times. It doesn't help matters that he's the only American in the cast surrounded by big-name British actors, including Irish born Peter O'Toole.
Jim Carrey in any of his earlier movies. While his characters are not outright stated to be American, one assumes anyway. Certain words are very glaring to his Canadian accent (e.g., "borrow," "progress"). To be fair, he either doesn't try to hide it or doesn't think to do so.
According to some accounts, Renée Zellweger averts this in the Bridget Jones movies. Her British accent was reportedly so convincing that when she reverted back to her natural Texan one after the first film wrapped, somebody asked why she had adopted such a "phony American accent."
Channing Tatum couldn't seem to make up his mind whether he had an accent or not in The Eagle. It goes back and forth from vaguely British to American. One can only assume he was going for The Queen's Latin.
Happens in Your Highness. As they enter the labyrinth, Thaddeus speaks a sentence in a clear "standard American" accent, as opposed to the British one he had the rest of the time.
In Scream 2, Randy inexplicably has a faux-British accent during his first scene.
Parodied in The Producers (the remake) when Max Bialystock calls himself O'Bialystock and fakes an Irish accent, which gets progressively worse as he continues talking.
Max: And now I'll be on me way, before me voice gets any higher!
In Dr. Strangelove, British Peter Sellers' portrayal of American President Muffley noticeably lapses into English pronunciation during the doomsday scene. The supposedly Soviet Ambassador (played by the also-British Peter Bull) was a lost cause from the beginning.
Back when Sellers was still supposed to play Major "King" Kong, he complained of the considerable trouble he had affecting a Texan accent and likely would have run afoul of this trope had the role not been recast with Slim Pickens.
Pierce Brosnan's usually convincing English accent slips frequently in the hospital scene of Die Another Day. Particularly obvious examples are his liquid consonants in the lines "I never ask to be traded" and "number one is no deals", and pretty much any instance of the unrounded "A" sound (as in "partner"), as well as a few other vowels.
Toby Hemingway is British. His character Oscar in Feast Of Love is American. His accent wavers a bit sometimes, particularly in the scene of Oscar and Chloe in Oscar's bedroom, where they discuss his past.
Die Hard: British Alan Rickman's German accent as Hans Gruber vanishes when he delivers the line "Blow the roof!"
Katie Cassidy slips back and forth between a Texas accent and normal American accent in Monte Carlo.
Miley Cyrus had to get a vocal coach to keep her from using her natural Tennessee accent (although, it seems to be less noticeable since she's moved to Los Angeles) in The Last Song. While she generally managed to not sound Southern, she never managed to keep any sort of New York (where her character was from) accent for long and in some scenes, you could definitely hear her true accent, usually the more emotional ones. The Australian Liam Hemsworth who plays Miley's character's Georgian (as in the state, not the country) love interest, seems to avoid this trope entirely, though.
Sir Alec Guinness in Cromwell as King Charles I. The character deliberately puts on an English accent to cover up his natural Scottish accent. The Scottish comes out when the character gets angry or isn't surrounded by courtiers. This can be considered a case of Fridge Brilliance because Charles I was actually Scottish - born when Elizabeth I was still alive and his father was just the King of Scots, he moved to England at age three.
Simon Baker struggles with his American accent at times in Margin Call. He doesn't quite slip back to his native Australian, but he acquires an odd brogue for some lines.
British actress Emily Lloyd's Brooklyn accent in the 1989 film Cookie was considered so poor that, for the next movie where she played an American, In Country, she went to live with a Kentucky family for a long time before shooting started. As a result, her accent in that film was much better.
In Who Framed Roger Rabbit, Englishman Bob Hoskins does a fantastically convincing New York accent for the character of Eddie Valiant (which is strange, as Valiant lives in Los Angeles, but then East Coast is probably an easier accent for the British to do than Californian). It does, however, slip a few times: most notably, when Roger hides in his desk drawer, and Eddie screams "GET OUTTA THERE", sounding less like a hard-boiled American detective and more like a British West-country farmer. In this case, it's not the pronunciation so much as the inflection; most Americans would put the emphasis on "outta", but Hoskins as Eddie puts it on "there". In the same scene, when Eddie says "Not anymore" (in response to Roger telling him "There's only place to go: Valiant & Valiant.") and "Don't ever" (as in "For starters, don't ever kiss me again."), he suddenly sounds more like Miles O'Brien than Sam Spade. Later, the accent slips again when he says "murder" to R.K. Maroon (as part of his line, "A story of greed, sex, and murder.") when pretending to. Simply put, Bob Hoskins seems to have trouble with words ending in the "r" sound.
Matilda: Playing Miss Honey, South African actress Embeth Davidtz is absolutely unable to convincingly fake an American accent. At times her inflection gives the game away, such as her use of "NO-b'dee" instead of "NO-buddy." Mrs. Wormwood's ear-grating Long Island accent also fades out at times (Rhea Perlman only has a slight Brooklyn accent in real life).
Rachel Weisz's American accent in The Fountain is, for the most part, passable, but there are moments it goes straight up her nose.
In the 2011 film Warrior: In the scene where Paddy visits Brendan for the first time, Joel Edgerton's Aussie accent slips out when he says "I've got a wife and kids; I don't have time for whatever this is." (It's adorable.)
In Fight Club: in the scene near the end where the protagonist tries to explain the Tyler situation to Marla, Marla says: "I tried Tyler...", and Helena Bonham Carter slips into her native accent when she pronounces the name; she noticeably aspirates and broadens the starting vowel.
German actor Horst Buchholz pulled off a credible Polish accent in most of Tiger Bay, only to stumble when his character started yelling near the climax.
In Les Misérables (2012), Sacha Baron Cohen's accent is all over the place. Possibly intentional, as he's playing a con artist who adopts several identities over the course of the story. In particular, his occasional French accent (the only one that any actor in the film attempts, even though the film takes place exclusively in Francenote everyone else speaks with a British accent.) is clearly an affectation that the character adopts whenever he wants to sound "classy."
Crowe brings a sleazy, blustering charm to the part of the villainous mayor, even if his accent racks up frequent flyer miles careening between New York, Boston, and Sydney.
Aliens. Carrie Henn, who played Newt, was living in the UK at the time of filming (one of her parents was British, the other American), and apparently picked up some of the accent. Near the end when the protagonists are escaping through the air ducts, she suddenly slips into an English accent on this line:
Newt: Up there! There's a shortcut across the roof!
In sex, lies, and videotape, Laura San Giacomo's apparent difficulties in covering up her native New York metro-area accent with a Southern drawl result in her either barely using it in some scenes, or overdoing it slightly in others.
In Suicide Squad (2016), Margot Robbie lapses noticeably into her native Australian accent. Most noticeable in the scene when the first nanite bomb is detonated, in the line: "Well, you better make it quick, 'cause he's gonna kill all of us one by one". Though at least there's the justification that she's playing a complete nutcase, so speaking weirdly is par the course. However, she did get better at it, as her accent is much more consistent in Birds of Prey (2020).
In Ghostbusters (1984), both Dan Aykroyd and Harold Ramis fail to conceal their natural accents in various places. Egon Spengler sounds awfully Chicagoan for an Ambiguously Jewish character supposedly born and raised in Cleveland and currently living in New York, and for all intents and purposes, has probably been living in New York for several years. Similarly, for someone purportedly born and raised on Long Island, Ray Stantz sounds very Canadian. It's notable at the end when Ray's apologizing to Peter for his "barbecued dog hair" comment; "sorry" comes out as "soary" as it does at various other points in the film.
Bill Murray as Peter Venkman seems to play this trope in reverse: mostly he sounds as Chicagoan as he normally does, except for a couple of scenes where he seems to be trying on something vaguely Brooklynese. "I believe we were destined to get thrown outta dis dump" and "I guess they just don't make them how dey used to, huh?" being the main examples.
Sharlto Copley as King Stefan in Maleficent. While the film's setting is not explicitly stated (the novelization says Scotland, but the film itself makes it out to be a standard fairy tale medieval kingdom), everyone has some kind of British accent. Copley, a South African, does his best to sound Scottish, but viewers were not entirely convinced. Especially noticeable since his wife In-Universe, Leila, is played by British actress Hannah New, who naturally has such an accent, while his daughter, Aurora, is played by American actress Elle Fanning, who manages to sound convincing enough.
In Whip It, Alia Shawkat's difficulty in maintaining her Arabic accent audible in some of her deleted scenes resulted in director Drew Barrymore instructing her to speak in her natural American accent for the remainder of filming.
Although Johnny Depp puts on a British accent as the Wolf, he drops it completely once he starts singing.
As the Witch, Meryl Streep seems to switch between British and American at certain points in the film. Given that Streep is, by far, the past master of acting with foreign accents in Hollywood, this is as likely as not to have been deliberate.
Lucy Punch sounds virtually American when singing, but retains her natural British accent when speaking. Considering all the other actors portraying the characters in the Cinderella story are American, this was probably an attempt to avoid standing out.
In Big Eyes, Christoph Waltz's own Austrian tones sometimes come through, particularly when he's shouting, such as in the scene when Margaret confronts Walter about "S. Cenic"'s name being on the paintings that he claimed he did.
Sebastian Stan has mostly done away with his original Romanian accent, but you can hear it very slightly in The Education of Charlie Banks when he says "I'm way too hungover for a lecture" right before Mick (Jason Ritter) beats him up.
In the B-MovieWerewolf (featured on MST3K) both the love interest Natalie and the antagonist Yuri gain and lose multiple accents throughout the movie. This—and the the dialog's bad grammar—culminates in the riff:
Tom Servo: (imitating Natalie) Paul! You is a waar-wilf!
The 1997 Batman & Robin film had American actress Alicia Silverstone play a supposedly English character. Unfortunately, her accent was not at all convincing (though that implies she was attempting one at all).
Mel Gibson's mixed accent sounds slightly Australian throughout the first Lethal Weapon movie, especially on vowels and when he raises his voice. However, you could argue that it is in-character since it's perfectly logical that Riggs could have spent some time in Australia. It's faded by the third and fourth movies.
Jennifer Lawrence and Josh Hutcherson in The Hunger Games occasionally speak with their native Kentucky accent. However, this is completely logical since District 12 is supposed to be in the Appalachian coal-producing region, i.e. Kentucky and West Virginia, and this may, in fact, have been intentional on the part of filmmakers.
While Irish-born Colin Farrell can do a very convincing fake American accent, in Minority Report, his character Danny Witwer mentions seeing his father get "shot on the steps of our church in Dublin". The line had been added in as a failsafe if Farrell's accent slipped. This is played with in the episode of Scrubs where he appears. At first, he fakes a convincing American accent, claiming to be from Boston. Then he psychs JD and Turk, switching to his Irish brogue and speaking this way for the rest of the episode. His accent is really good in Fantastic Beasts and Where to Find Them but he noticeably slips when he says Credence will be honored among wizards.
Lisl: Ooh, me nightgown's slipping. Bond': So is your accent, your highness.
Inverted in Casino Royale (1967): Agent Mimi, played by the Scottish Deborah Kerr, impersonates M's widow, but upon witnessing the prowess of David Niven's Bond, doesn't lapse back into a native French accent, but starts singing his praises in orgiastic French.
In Shadow of the Thin Man (1941), high-class prostitute Claire Peters usually spoke with a pronounced "posh" British accent (or at least what sounded like one to Americans). But when frightened or angry, she would slip into a lower-class New Yorker accent. Nick catches her doing this in one scene.
In Cromwell, King Charles (Alec Guinness) disguises his Scottish accent, until one scene where he memorably loses his cool. Of course Charles's father was Scottish so it seems reasonable his son might have picked up traces of it despite spending the vast majority of his life in England and being surrounded by people actually born there.
The British Michael Caine played a Maine doctor in The Cider House Rules. In case his accent slipped, the director justified it by having the doctor mention his mother was an immigrant.
Layer Cake has this with Eddie Temple (played by Michael Gambon), that's a combination of this and the first type. Gambon's character often speaks with a plummy English accent, and the director's commentary mentions his complaint about difficulty holding the accent. The director told him to let the accent slip at will any time he felt like it, which fits the character well, a London Gangster who has moved into high society and taken elocution classes. In particular, his posh accent tends to slip when the character is angry.
In Velvet Goldmine, Toni Collette plays an American woman who moves to England and subsequently develops a British accent. In flashbacks, her accent slips when she's upset; in scenes taking place in the present, she doesn't bother. It's particularly impressive given that the actress is Australian, therefore neither of the accents she veers between are her own.
This is actually a positively uncanny imitation of Angela Bowie, who is American but either absorbed or intentionally adopted a faux-British accent during her then-husband's glam rock days. It's especially prominent in the opening to Ziggy Stardust: The Motion Picture, for example, when she comes backstage to chat with the band.
A deliberate regional-American-accent example occurs in The Wizard of Oz, when the fortune-telling huckster speaks to Dorothy in the tones of an educated East Coast man, then slips into a rustic vernacular to talk to himself after she's left. Beautifully done by Frank Morgan.
In Tenacious D in The Pick of Destiny J.B. when falls prey to the street gang that is a Shout-Out to A Clockwork Orange. Its members speak with mocked-up British accents and when one of them utters a phrase with the actor's natural accent, he is promptly punched to the chest by the leader and returns to the mocked English.
In Pacific Rim, Pentecost's accent goes on and off depending on his tone of voice, indicating that his heroic front is, to some degree, a façade. He drops it entirely when he tells Raleigh about the radiation poisoning.
Part of The Reveal in the movie Gotcha! While arguing about whether she used him or not, Sasha's accent slips and drops completely when she admits she isn't from Czechoslovakia.
In 7 Faces of Dr. Lao, the eponymous wizard casually slips back and forth between speaking with a heavy Chinese accent and deploying perfect English diction, a fact which is explicitly noted by one of the other characters and lampshaded:
"Oh, it comes and goes. Whatever dialect the mood requires."
Russell Crowe keeps slipping back into his natural accent in The Nice Guys. Of course, it's not explicitly stated that he's supposed to be American.
Inglorious Basterds: When the English Lt. Hicox goes to rendezvous in a bar with German actress/spy, Bridget, hes impersonating an SS officer. An SS officer becomes interested in their table and notes that everyone at it has an identifiable regional accent except for Hicox, whose accent is strange and unfamiliar. He casually claims that he's from an extremely remote Swiss village (one documented in a famous Leni Riefenstahl film, no less), which explains why his accent is both strange and unfamiliar to Germans.
Robin Williams adopts the guise of an old Scottish nanny in the movie Mrs. Doubtfire. To North American viewers the accent is likely pretty convincing, but a supposedly British character (played by Irishman Pierce Brosnan) lampshades "her" constantly-shifting accent which is much less convincing to British viewers, and asks where "she" comes from. Williams's character hastily improvises to say she's from everywhere, moving all around the country growing up and adopting bits of the local accents as she went.
Probably done purposefully in The Avengers (2012). Despite the character of Natasha Romanov being Russian, Scarlett Johansson almost never uses a Russian accent (the scene where she's actually in Russia talking to a couple of Russians being the most notable exception). However, she does get one minor nod to the character's heritage during the Battle of New York. When Natasha makes the observation that the Chitauri invasion reminds her of a previous mission they were on, Clint says "Budapest" how it's spelled, like a typical American would. Right before that, however, Natasha pronounces it with an Eastern European-style "Budapesht".
Intentional in Unforgiven. Richard Harris speaks with a posh London accent most of the time, as befits the upper-class persona English Bob. Then his character realizes he's facing "Little Bill" Daggett...and his accent slips, revealing his lower-class upbringing: "(Cockney) Shit and fried eggs. (Posh) Well, hello Bill. (Cockney) They said you was— (posh) They said you were dead. That you fell from your horse, and broke your bloody neck. Then as he's leaving town, he curses out the people of Big Whiskey in Cockney, all pretense gone in his humiliation and rage.
"Hell naw, I did not leave the South Side for this!" <sets off the fire alarm> "ALL JUNIOR GIRLS REPORT TO THE GYMNASIUM IMMEDIATELY!"
In Singin' in the Rain, Lina Lamont's inability to consistently maintain the refined accent her role demands even after receiving diction coaching is part of what turns The Dueling Cavalier into Stylistic Suck.
Beauregard: It takes a while to get to know the town! Kermit: How long have you lived in London? B: Aaaall my life! Fozzie: How come you don't have an English accent? B: Hey, I'm lucky to have a driver's license!
Highlander: The Frenchman Christopher Lambert, in spite of being born in America, spoke almost no English when cast for the role of immortal Scottish highlander Connor. In fact, the director has claimed that he more or less recited his lines phonetically. As such, he has a great deal of difficulty maintaining anything close to a Scottish accent. There's no excuse for this in the scenes taking place in Scotland, but in the scenes in America, several hundred years later, his muddled accent actually makes sense. When a character challenges his character, "You talk funny" and asks where he's from, Connor replies. "Lots of different places." Ironically, Lambert plays opposite Sean Connery, who makes no attempt to adjust his natural Scottish accent when playing a Spaniard from ancient Egypt.