You've seen it a million times. Character A is approaching a door, or is standing beside or in front of a door, when abruptly the door is thrown open from the other side by Character B. Character A is then revealed to have been squished against the wall by the door when it was smashed open.
While this is a type of Slapstick that is very common in animation, being hit by a door happens in live-action media also (not to mention real life). Definitely a case of Truth in Television: if you've ever seen a sign on a door warning you to open it slowly, chances are that sign was put up after someone got clobbered by that particular door.
There's also a second variant, where someone is rushing at an open doorway for whatever reasons he or she may have ... only to have someone else close the door. This trope applies when the runner ends up splatted on the door; if they make it through, it's normally Impact Silhouette, and if the runner dives through a narrowing gap, you have an Indy Hat Roll variant.
Compare with Door Judo. See also Toyota Tripwire. Might be the deed of a Ghost Butler, or a side effect of Tactical Door Use. Can also overlap with Face Plant.
In a similar vein, Japanese tourists are photographing a Dutch windmill when Leslie Nielsen suddenly opens the windmill's door, knocking the miller into the path of the sails. Nielsen then proceeds to explain the benefits of Dutchtone, unaware that the tourists gasps of amazement are over the miller traveling round and round on the windmill sails.
Anime & Manga
An early chapter of Mahou Sensei Negima! has Asuna kicking the library door off its hinges, and into Negi.
One of the few times that Shampoo uses the door to enter the Tendō house, she flattens Akane behind it.
Happens again to Akane when Ranma, in order to satisfy his mother's ideals of "manhood", burst into the bathroom to (pretend to) spy on her. Too bad Akane had come out several minutes before and was slammed quite painfully between the door and the wall... and hearing that Ranma wanted to see her naked did not improve her day.
And Ranma also did it to Kodachi in her own intro story just as the latter was about to attack Akane. She just twitched and spasmed behind the door for a while.
Sort of happens in the anime when Dr. Tōfū is introduced; startled by the good doctor appearing behind "her", Ranma leaps up onto the door. Akane comes barging in and slams the door shut, catching Ranma's fingers painfully between the door and the edge of the wall due to Ranma not having enough time to let go before being knocked out of the room.
Done in Naruto, when Suigetsu goes towards a door to try and find where Karin went... right as she gets back from finding there's a Leaf Village party headed their way — and kicks down the sliding door on his face.
One Piece has this happen to Apis, who tries to escape the custody of the Marines by tricking the guard into opening the door by not being in plain sight of the door. Problem is, her hiding place was the side of the door that it opens into.
An episode of the Black Cat anime has this happen to Sven as he's going out for a walk and Rinslet slams the door open just as he reaches it.
In book 4 of Futaba-kun Change!, Principal Hiroin in one of his silly "Justice Maker" costumes is about to make the usual corny speech, when his daughter Takane slams open the door he was standing next and sends him down a flight of stairs (she doesn't notice anything except the broken stairwell her father left behind as he fell down a few stories). And then Futaba-chan comes by in a hurry and slams the door against Takane as she was starting her own speech.
Hanaukyō Maid Tai La Vérité episode 7. As Konoe Tsurugi is waiting on one side of a door, her lieutenant Yashima Sanae slams open the door from the other side and smashes her in the face, giving her a temporary bloody nose.
In an episode of Urusei Yatsura, Ataru is trying to get out of his house and away from Lum, and slams the door in her face as he's exiting. An upset Lum pushes the door forcefully open and flies out to catch up to Ataru, but can't see him anywhere. A zoom in then shows that Ataru is flattened behind the door, his face ground into the wall.
Both Daichi and Mii gets slammed into the wall by Mai opening the front door at the wrong moment in the first episode of Popotan.
The leads of Victorian Romance Emma, Emma and William, first meet when Emma opens the door right into William's face as he's raising his hand to knock.
In Astérix and Cleopatra, Obélix does warn an Egyptian prison guard that he and his fellow Gauls are about to come out of their cell, but the guard just laughs it off, thinking he's safe sitting behind the heavy reinforced door. Not so, which result in him being slammed against the floor, and Obélix grumbling he should have listened. Repeated both in the Animated Adaptation and the Live-Action AdaptationAstérix & Obélix: Mission Cleopatra.
Pooch Cafe for 5-26-11. Chaz warns Carmen to not stand near the door because Poncho will be coming through like a train. She doesn't believe him, and gets smashed behind the door when Poncho slams the door open and charges through with a "Choo Choo" sound effect.
The Garfield comics and cartoons. Notably, one Quickie has Garfield hanging on the door when Jon opened it...
This happens to Jafar and Iago both after Jafar tries to hold the palace doors shut to keep "Prince Ali" from entering the Sultan's throne room. Abu (in elephant form) smashes the door open and squashes them into the wall.
It happens earlier to Iago when he and Jafar are walking out of their secret lair. Iago is about to cross the door/wall when Jafar has to shut it when Jasmine pops out of nowhere, smashing Iago in the process.
In The Jungle Book, while Bagheera is beside a door in King Louie's ruins and trying to grab Mowgli so they can escape, Baloo slams the door open and smashes Bagheera into a wall, giving him a black eye.
In the (still animated) beginning of Enchanted, Nathaniel goes to open Giselle's coach, and gets slammed by the door as she bursts through.
Cruella is in her car in the street, and yells for Jasper and Horace, who are inside a wooden building with a locked barn-style door that leads out to the street. They try to break down the door, and they knock it down right into her face, as it turns out she was parked right outside.
It does happen earlier in the movie too; as Cruella comes to visit the Radcliffes, Nanny walks over to open the door for her. Cruella manages to open it on her own.
In Twice Upon a Time, Greensleeves gets pinned under the door to his prison cell when Rod Rescueman knocks it down.
Done in Transformers: The Movie to a Quintesson. The Dinobots knock the door to the execution room down on top of him. And then as Slag (the Triceratops) and Sludge (the Apatosaurus) walk across the door, they stop for Slag to say, "Excuse me."
Films — Live-Action
In Trail of the Pink Panther, Inspector Clouseau is knocked out a window by a chambermaid opening the door. Three times in a row.
The Three Stooges often did this. In one especially memorable example, a freestanding door the boys were installing fell on top of Moe and got jammed, and Larry and Curly had to saw it to get him out. The saw cut the floor and sent Moe falling 14 feet to the basement. Larry doesn't see why Moe is so miffed. On occasion they'd trap the villain's head in or through a door, and, once certain he couldn't extricate himself, proceed to add to his misery.
In Home Alone 2, the two crooks hear a noise on the other side of the door. It's a heavy toolchest, which smashes them and the door against the opposite wall. In utterly realistic fashion, they only get their noses bent.
In the Mighty Morphin' Power Rangers movie and show, Adam disposes of one of Ivan's grunts through the means of a construction yard vehicle's door.
Adam: Let me get the door!
This is a running gag in The Naked Gun series, with Frank Drebin doing it at least once per movie. He tragically knocked his first girlfriend off a cliff (then never realized and thought she'd abandoned him). He also did it to Barbara Bush when invited to the White House for a dinner. He also does this to a gun nut who was holding the entirety of Police Squad at bay (and was totally oblivious once again).
In Back to the Future Part II, one time-traveling Marty does this to another time-traveling Marty.
In a rare non-comedic example, The Fellowship of the Ring has this when the Nazgul burst into Bree. The terrified door guard looks through the door, and gets it slammed off its hinges, on top of him. Ouch.
The Three Musketeers (1973). The Duke of Buckingham slams open a secret door, hitting Planchet who's standing behind it.
In Clerks II, Jay twice gets hit while trying to take a leak next to the back door.
In Toy Soldiers, the Big Bad rushes into the kitchen looking for one of the Kid Heroes. The kid in question hides behind the kitchen's swinging door, and for a moment it looks like it's worked. And then the Big Bad grabs the door and slams it into the wall, crushing the kid and knocking the wind out of him.
In the first Johnny English movie, this happens twice to the eponymous hero. First when Johnny is about to ambush a henchman in the Big Bad's tower, and gets hit in the nose by the door. And again when tied up and locked in a cell, while trying to open the lock by whistling. His Hypercompetent Sidekick Bough opens the door to free him, as Jonnhy is leaning right behind it.
Happens to a security guard in Freddy vs. Jason, with predictably fatal results.
The original The Nutty Professor opens with the title character blowing up his classroom. As another teacher anxiously looks for him in the wreckage, she hears a knock on the door she's standing on - and finds him underneath.
Teal'c gets a door in the face in the episode "Windows of Opportunity". Repeatedly, as he is stuck in "Groundhog Day" Loop and this happens right at the beginning of it — he has no chance to avoid it, every time, for hundred of loops. And then he slams back.
One "missing scene" fanfiction has Teal'c taking his revenge during one loop by bringing an electric saw and cutting the door into tiny pieces.
Happens a few times in The Nanny to Niles, CC and Fran. Especially CC, since she was usually found sitting on a leather couch behind the door in Maxwell's office.
Cindy in Threes Company constantly did this inadvertently to the rest of the cast; so much that Jack tried to teach her in one episode how to open a door slowly.
In Leverage, Eliot Spencer (who else?) uses this as an attack. He was fighting a group of drug dealers, one of whom was in a truck repeatedly trying to get out to help his friends. Every time he'd try, Eliot would slam the door on him, at one point simply yelling "stay in the car!" which apparently intimidated the man enough not to even try the other door.
A knock comes from the door, and says Blackadder to Baldrick:
Blackadder: See who that is Baldrick. And whoever it is, slam the door in their face, or I'll slam your face in the door.
Unfortunately for Blackadder, it was Queen Victoria and Prince Albert.
Happens to Erin a couple of times in the Titus episode, "The Visit", eventually leading to child-services officer to say, "Want a hint? Hold the door for her!"
1960's Batman series episode "King Tut's Coup". While Robin is waiting beside the door to Tut's hideout, two of Tut's henchmen slam the door open and smash Robin against the wall, knocking him silly. They then capture him and take him inside.
In the Buffy the Vampire Slayer episode, "Lovers Walk", Angel gets crushed under a door while attempting to defend Spike. A few mooks walk over him shortly thereafter.
In the episode of The Goodies entitled "The Movies", Graeme manages to inflict one of these to himself — he kicks open a door with enough force to have it bounce off the wall and slam in his face.
The Two Ronnies. In a spoof of The Professionals, a gun-waving Doyle kicks open a door, searches an apartment and is surprised to find no-one there. As he closes the door behind him, a Squashed Flat villain (represented by a rubber dummy) falls out from behind it.
In Monster Bash, the Mummy does this to one of the archaeologists standing in front of his sarcophagus.
MAD's Super Special Fall 1980. In "The Moronic Woman" (a The Bionic Woman parody), Jammy Summons (Jaime Sommers) and Oscular (Oscar Goldman) have infiltrated the enemy palace. When they reach the office of the dictator, Jammy slams open the door with her moronic (bionic) strength. When Oscular asks where the dictator is, an officer says that he was behind the door, so he's now part of the wallpaper.
This happens to Bean Bunny during the breakfast scene in Muppets from Space. The culprit is Miss Piggy, who always has to make a huge entrance, Large Ham that she is.
The victim: (moments before) I finally got over my fear of standing next to closed doors! (WHAM!) Uhh!
Happens twice in British farce Cash on Delivery to the Uncle, setting up the comic misunderstanding first that he's crazy (due to him being dazed) and then dead (because he's been knocked unconscious).
In Hotline Miami, Jacket opens the door with a slam, which can knock a mook down if they are hit by it. Better; There's an unlockable mask that lets you kill mooks by hitting them with the door slam.
You can keep using doors, after they've been used, so you can always cause a door slam.
Resident Evil 4 & 5 allow you to either open doors slowly and quietly in order to stealth in, or loudly kick the door open. If you use the more Dynamic Entry then any enemies standing on the other side will be knocked back.
In Luigi's Mansion, some of the doors are fake. When Luigi goes to open them, they'll swing around and crush him against the wall.
In Paper Mario, the first time you go to see Merlon, he knocks you down when he opens it.
In Paper Mario: The Thousand-Year Door, when you're working your way to the top of the Glitz Pit rankings, you let the promoter know when you're ready for the next fight, then wait for security to come get you. If you're in front of the door, you flail about as you're pushed out of the way.
In the 3D Legend of Zelda games, fake doors crush Link against the floor by falling on him.
In Splinter Cell: Chaos Theory, a guard can kick open a door if something is blocking it. If that something is you, then the door will knock you out causing an instant mission failed. Conversely, you can kick open doors and knock out anyone else who is blocking it from the other side.
This is the main form of attack of the eponymous mouse policeman in the Arcade/NES classic Mappy.
Some early video games with doors that actually work like real-world doors, rather than Doom-esque upside-down-elevator doors, can crush the player if they stand in the wrong place as it automatically closes.
In Brain Dead 13, this happens to the centipede in the wine cellar, except that when it chases Lance towards the exit, he opens the door and intentionally slams it into its face, putting it out of commission.
Quest for Glory I on two occasions. Standing in front of, or to the wrong side of Henry the Hermit's door when he opens it will send the hero flying off a cliff, or crush him between the door and a cliff face (he does warn you to stand clear). During the brigand hideout sequence, there is a fake door that will fall down and also crush the hero if you don't get out of the way.
A gameplay mechanic in Gunpoint. Wire a door just right, and it'll smack a rent-a-cop in the face. This knocks them out. Wire a door just wrong, and it'll smack you in the face. This gives you an achievement. Jumping kick a door off its hinges, and it can hit a rent-a-cop in the face. This knocks them out and gives you an achievement.
In an episode of The Fairly OddParents, Timmy and Wanda are invited to lunch in fairy world by Mamma Cosmo, but the two are seated at a table situated between the kitchen and the bathrooms. Cue the two getting repeatedly smacked by the doors as fairies enter and leave.
In the first episode, Twilight Sparkle does this to Spike in her hurry to get to the library.
Happens to Twilight twice in "Feeling Pinkie Keen", both of which Pinkie Pie predicted.
Happens to Fluttershy when she tries to hide in her house (again) in "Luna Eclipsed" thanks to Twilight preemptively closing the door.
In "The Best Night Ever", Prince Blueblood does this to Rarity after making her get the door for him.
One The Pink Panther short has the inspector declaring, "All right, Pink Panther, I know you are in there; come out at once!" only to get a door in the face. And no matter where he stood, the door opened on him....
In The Legend of Korra, Korra is treated like a pinball within the confines of a 2000-year-old Airbending training device.
In Futurama's "Space Pilot 3000", Fry stares in awe of the automatic sliding doors that slide up. However, he's doing so directly under the door, so just as he says "Wow, just like in Star Trek" the door closes down on him.
Happens to Helga on Hey Arnold! in the episoded Beaned. Arnold is holding the door open for everyone to enter the cafeteria because they're holding their trays and Helga assumes he'll keep it open for her as well. Just as she's about to walk through, he slams it shut on her (unknowingly), slamming her tray and all its' content into her face as she slowly slides down to the floor.
The title character of Dan Vs.. gets one in "Dan vs. Canada" when, thanks to an unfortunate maple-syrup-related accident, he becomes stuck to his own apartment door. After calling Chris for help, Chris arrives and slams open the door along with Dan squashed by it.
In the Tiny Toon Adventures episode Europe in 30 Minutes, a door is slammed on Buster, Babs, Plucky, and Hampton, flattening them from the front.