Language can cause confusion even when two people aren't using separate languages. Often, it's enough for them to speak different dialects of the same language. Most of the examples here are about the differences between British English and American English, but this can occur in any pair of dialects, in any language.
Confusion over dialects in Real Life varies from person to person, and depends on the situation. If British Alice says "throw it in the boot" to American Bob in a story, without context he may be confused; but when they're obviously near to a car, it isn't a huge leap that she means that something should be put in the "trunk".
It can also be a giveaway when a writer sets a story in a land besides his native one. Many English and Canadian writers write for American audiences. Often they're so good at it that the reader doesn't notice—until a Texan character mentions something that happened "when I was at university." (An American would say "when I was in college" or "...in grad school", even if he got his degree at a university.)
People who read or watch a lot of fiction in another dialect will understand more words than expected, which may even be Lampshaded by the writers. Before the rise of the Internet in the early 1990s it was posited that the different versions of English might eventually diverge into separate languages, however these days the Internet has made it possible to inexpensively speak to a friend in a like-language country on a daily basis, so time will only tell.
See also Australian English and Did Not Do the Bloody Research. Contrast Language Barrier, which is Separated By Lack of a Common Language.
Do not add Real Life examples here. For Real Life examples, see the Useful Notes version of this page, Separated By A Common Language.
Examples in fiction and media:
"Knocked up" gets used in Supergirl, when the British-accented demon Buzz asks Linda to knock him up in the morning. Eventually they get it sorted out.
One Buffy the Vampire Slayer Season 8 has Faith go undercover to defeat a rogue slayer. The other slayer asks Faith "Do you mind if I bum a fag?" It takes Faith a moment to realize it wasn't some kind of Double Entendre.
An eighties Justice League comic had The Flash (who had no Secret Identity at the time) introduce himself as "Wally" to a group of English girls, who respond with derisive laughter. Afterwards someone explains to him that "Wally" means an idiot. It's a very gentle insult, like something your nan would say. Using it non-jokingly would be seen in a similar way to Have a Gay Old Time.
Japanese example: in Detective Conan, the different meanings of horu (see Useful Notes page) were used to identify the criminal who came from Osaka.
In Paul, two British comic-book nerds attend Comic Con in San Diego and then go on a road trip in the American Southwest. Clive asks where they parked the RV.
In America, "pissed" means angry; in most of the rest of the English-speaking world, it can also mean "drunk", while angry people tend to be described as "pissed off" rather than just "pissed". On the commentary for Shaun of the Dead, it's stated that the only thing they consciously did to avoid confusing Americans was to say Mary the zombie was "so drunk" instead of "so pissed", because they were aware of this trope and knew how that would sound.
The meanings of "pissed" led to a memorable scene in the Bob Dylan documentary Don't Look Back. In a hotel in London, Dylan (who is of course American) threw a fit when someone threw a glass (or was it a lightbulb?) out the window. The guy was drunk and tried explaining that he was pissed, so Dylan said something along the lines of "Oh, you're pissed?! Don't tell me you're pissed!"
In "Money, A Suicide Note" by Martin Amis, the main character is sent to try to get a very clean cut actor called Spunk Davies to use a stage name instead.
In Good Omens, Newton Pulsifer has a Witchfinder's card which states that faggots (meaning chunks of firewood) should be turned over to him. When an American soldier asks what they do with the faggots, Newton replies "We burn them." The soldier is quite impressed...
Though most of these are now obsolete, G.K. Chesterton notices a number of these Anglo-American differences in his writings. He was told that a lady journalist had referred to him as "a regular guy" and did not see how that was meant to be complimentary; in the American usage of the time, the phrase meant "someone who was affable and not snobbish," whereas in the British use then current, it meant "a figure designed to be mocked, a scarecrow, or an object of ridicule." He was warned by an American friend against certain people because they were "very bad actors"; GKC did not know why their defective thespian abilities should cause him anxiety, until it was explained that this was American for "malefactors." Similarly, an American friend told him her sister had "gotten a beau"óby which he understood her to have devoted herself to archery.
Spike: (checking the label in his jacket pocket) "Made with care for Randy"? Randy Giles? Why not just call me "Horny" Giles, or "Desperate-for-a-Shag" Giles? I knew there was a reason I hated you!
Giles: Randy's... a family name, undoubtedly.
An episode of Have I Got News for You had lots of fun with the fact that there's a staffer in the Obama White House named Randy Bumgardener. To elaborate: Randy = horny. Bum = ass. Gardner = "Uphill gardener" (a rarely-used slang phrase referring to a gay man, the "gardening" being, well, y'know...
An episode of The Graham Norton Show in the UK had this confuse guest Johnny Knoxvile after an audience member tells Graham on how she stuffed a rubber up her nose. Johnny's reaction led fellow guest Catherine Tate to tell him it's an eraser. Graham was very amused.
Nicely played with in Blackadder Goes Forth, when Melchett assumes the crossdressing George is a woman and says she's "full of spunk." Blackadder is quietly amused.
This gag from The Office UK was made funnier by Keith's general creepiness and monotone delivery.
Keith: "Fanny" means your arse. (Beat) Not your minge.
Duncan refers to leaving his wallet "in the back of my lorry", presumably because someone heard that 'lorry' is British English for 'truck'. In fact it suggests that the psych professor mysteriously owns an 18-wheeler - pickup trucks are hardly ever seen in the UK, and when they are they're called 'pickup trucks'. Not to mention that, as shown in the pilot, Duncan doesn't have a pickup truck at all; he drives a Smart car.
Duncan also states that he grew up on "52nd Street in Islington". Assuming he means Islington in London, not only is there no such street, but in British cities streets are usually named, not numbered in the same way that they are in American cities.
In-universe example: "Let's blow this pop stand and head out back for a spot of slap and tickle. That's sex, in case the lingo hasn't made it across the pond."
...all of which are part of the larger joke that Duncan doesn't actually know any English slang and knows very little about England, having come to the US at a young age with his grandfather.
The word 'rubber' means an eraser, but in other places it's a condom. As a guest of his Late Night show, Emma Watson told David Letterman that she committed this infraction during her first semester at an American university (Brown), asking a male classmate for a rubber during class.
The difference in meaning of the word 'shag' turned into a Double Entendre in Blackadder Goes Forth (set in World War I), as General Melchett tells "Bob" that Captain Blackadder does a "good line in rough shag" and could "fill your pipe", though it might be invoked Have a Gay Old Time. Shag used to mean coarse-cut tobacco, but now it refers to sex.
'Shit' can be used in colloquial American or Australian English instead of 'stuff' or 'nonsense'. In the UK it remains much more of a negative word. The American usage is beginning to catch on in the UK, though it confuses some in the audience and some characters. It was used on How Not To Live Your Life:
Eddie: ...and shit?
"The Gambler Song" (a parody of the Kenny Rogers song "The Gambler") by Australian comedian/singer/songwriter Martin Pearson contains this lyric:
"Then he bummed a cigarette/
(Most people smoke them with their mouths)"
The differing meanings behind the word thong were used as a joke in the comic strip Zits. When Connie expressed a desire to wear thongs, it was quite the mental image for her son Jeremy.
The Bob And Tom Show: In "Cooking with the King", Elvis goes to England to try fish and chips and is completely confused by his host Ian St. Ian's speech. Eventually they get into a fight over the chips, which Elvis recognizes as French fries ("And they ain't even Ore-Ida's!").
Stand-up and Recorded Comedy
Jasper Carrott once built an entire routine around the idea of going into an American shop to buy a pencil eraser.
Jasper: Hello, can I have a rubber please?
Storekeeper: How many would you like, sir?
Jasper: Just the one! Gawd, I don't make that many mistakes!
Jasper: Oh! Have you got any of those ones with a Mickey Mouse head? Cos I like to chew on them when I'm thinking.
'Shit' can be used in colloquial American and Australian English as a substitute for 'stuff' (whilst retaining its specific, scatological meaning). In the UK it remains much more of a negative word. Stand-up comedian George Carlin observed this:
George Carlin: Have you noticed that their stuff is shit and your shit is stuff?
Russell Brand talks about an encounter with this trope in his New York comedy special, describing the time he was introduced to LL Cool J.
"'Yo, yo, yo, wazzup my homies?' — 'Oh yes, you're wonderful. I love all your homos.'"
'Shit' can mean 'stuff' in American or Australian English, though it's still connected with the original meaning. In British English it remains much more of a negative word. Stephen Fry once pointed out that if an American says: "I was eating hamburgers and shit", you can tell what he means. But written down it looks like he's saying, well: "I was eating hamburgers and shit." Ew.