Guilt By Association Gag aka: Did Not Eat The Mousse
Geoffrey: How did we all die at the same time? (The Grim Reaper points at one of the platters on the table.) Grim Reaper: The salmon mousse. Geoffrey: Dearest, you didn't use canned salmon, did you? Angela: I'm so dreadfully embarrassed! (much later, as they're all being carried away to the afterlife) Lady Presenter: Hey, I didn't even eat the mousse!
In a comedy, when a bunch of characters are subject to some kind of punishment or awful revenge, there will often be exactly one character who doesn't deserve it. No matter how much this character voices his objection, he will never be recognized as an exception. He must suffer with everyone else.
Compare: My Friends... and Zoidberg, Can't Get Away with Nuthin' , Go Among Mad People. If the character didn't do the listed thing, but DID do something else, may become I Take Offense to That Last One. The opposite of this trope is I Am Spartacus, where people who aren't guilty deliberately associate themselves with the guilty party and Accomplice by Inaction, where people who didn't do the act are considered guilty because they didn't do anything to stop it. Jokes of this kind are a type of Black Comedy.
This trope is NOT just any group of people where one person doesn't belong in the group for whatever reason.
A rare serious example: in Maus, the punishment comes from the Nazis towards a prisoner who may or may not actually be the race they claim he is. Every day at lineup, he desperately insists that he was a German soldier who doesn't belong with "these Yids and Polacks". This is emphasized when Vladek sees him as a cat (analogous to a German) but the other Germans see him as a mouse (analogous to a Jew.)
The titular death scene from Monty Pythons The Meaning Of Life. It was, in a rare case for Monty Python, an ad-lib. What made that particular scene even funnier is that the character sounded more excited than anything, since they were in the middle of an intellectual debate about the afterlife when Death arrived.
Another Monty Python example occurs in Life of Brian, during the crucifixion march scene. A kindly onlooker who tries to help a fallen prisoner by lifting his cross is marched off to the execution site by the guards after the prisoner ditches him and runs off into the crowd. This may be a particularly savage Take That at Simon of Cyrene.
Monty Python just loves this one— near the beginning of Monty Python and the Holy Grail, a man is loaded onto a cart full of dead plague victims. When he insists he's "not quite dead yet" and attempts to leave, the cart-pusher refuses to take him at first, but the man who brought him slips the cart-pusher some money. A hefty whack from his cudgel and the man wasn't complaining any more.
Narrowly averted in Dogma, when the two angels Loki and Bartleby slaughter the whole board of directors of a fictional corporation, for "raising an idol (a cartoon character) that draws worship from the lord..." They had all also committed other, horrible sins to deserve their punishment. The only one who was spared was the sole woman among them, who had committed no sins... though Loki still almost killed her because she didn't say "God bless you" when he sneezed. Bartleby stops him at the last moment, but only because he's tired of waiting.
In Chicago's "Cell Block Tango," five of the six women insist their victims "had it coming" while not accepting responsibility. The sixth protests that she is innocent; unfortunately, in Hungarian. She's the one who ends up being executed.
Live Action TV
On Arrested Development, Oscar is constantly being hunted down and savagely beaten by police (and was actually imprisoned between the second and third seasons) for the crimes committed by his identical twin brother. It's especially ironic because he's probably the only member of the family who hasn't committed massive fraud and "...maybe a little light treason"; he never seems to be guilty of much beyond possession of marijuana.
One episode of Father Ted features Ted and Dougal sneaking around after dark in a "rest home" full of sick and/or demented priests; the inmates all loom out of the darkness spouting deranged gibberish, except for one poor guy who enunciates quite clearly: "I really shouldn't be here!" He is of course left to his fate.
Non-Comedy example: In Ashes to Ashes the cops suspect someone protesting about the Docklands redevelopment might try something on the Royal Wedding day, so he rounds up members of the protest groups, and one man from the Anti-Nazi League by mistake; he gets the worst of it because he talks back.
Tom: Okay, okay, Mike, be honest with us, this music really kinda gets your blood pumping, right? Crow: Yeah, Mike. This is your music done by your people, so I blame you for this entire movie. Tom: Yeah, it's just like you to make a movie like this. Tsh.
Jack: General, if we did screw up their world, we should take a little responsibility for fixing it, don't you think? Hammond: I understand that. But let me play devil's advocate for a moment here. It's not our world. Is it really any of our concern? Teal'c: The destruction of the hammer device to save my life may have caused this. If so, I am responsible. Jack: General, I gave the order. Daniel: And I fired the staff at the machine. Sam: And I ... was there.
Turned Up to Eleven in the Flood story. All animals (except one breeding pair), newborns and even unborn babies were killed along with the "wicked" humans.
In Eight Bit Theater, the Light Warriors are all to be killed by Sarda because they're horrible people. Except for Fighter. "He's a casualty."
in Drowtales, this winds up happening to Kyo'nne when the Kyorl'solenurn arrive. She seemed to think she'd get off easily because unlike her friends she wasn't tainted and even mocks her friends for being "screwed" as they start to run, but the Kyorl capture her anyway for being "corrupted", literally guilt by association, and would have "cleansed" (read: brainwashed) her if Chirinide and Shan'naal hadn't interfered. For extra irony she was the only one the Kyorl actually managed to catch.
Warrior U: Finn gets all the class to skip school and go to the beach, where they are joined by recurrent antagonist Hevvin the Unicorn. After a while, they are caught by the teachers and locked in detention. Hevvin loudly protests that he shouldn't be there because he doesn't even to go that school.
In the episode "Fear of a Bot Planet", Leela objects to being sentenced as a human by the human-hating robots because she is a one-eyed alien. (Although before going, The Professordid warn her that the robots are "not fans" of humanoid aliens.) Turned out much later that she actually is human, albeit a mutant one.
Then another time Leela got fired with Fry and Bender when the two took the ship out for a joyride around the world with the Planet Express building tethered to it, but she did leave the key in the ignition.
In Metalocalypse, Toki is often the one who doesn't eat the mousse. One of his catch phrases is even "Evens Toki?" When Dethklok comes out with a line of clothing, their humanitarian fashion designer berates and abuses them all for not being in shape, including Toki, who is in great shape and who didn't lie about his measurements. (At least his new clothes fit perfectly.)
The Simpsons: in the "Heck House" segment of episode "Treehouse of Horror XVIII", Bart, Milhouse, Nelson and Lisa are sent temporarily to hell, yet Lisa took no part in the chaos they wreaked. She points this out along the way to hell: "I warned you, and yet my punishment is no less severe. That's odd".
In a different episode, "Simpsons Bible Stories", ends with the Rapture. The Simpsons are to be sent to Hell...except Lisa, who is pulled heavenward in a beam of glorious light. Disturbingly, Homer is able to reach up and pull her down to hell with them saying "Where do you think you're going, missy?"
In another episode, "Grade School Confidential," Martin Prince's parents throw him a birthday party...but they serve raw oysters instead of cake. The children all come down with food poisoning. Including, at first glance, Lisa (a vegetarian). However, it's a subversion in that she only pretended to be sick so she wouldn't have to stay at the disaster of a party.
In yet another Halloween episode, everyone in town is cursed by a witch, causing them to turn into their Halloween costumes, including Hans Moleman, who turns into a mole despite not wearing a costume.
In the middle of his long-repressed rage rant during "Hurricane Neddy", Lenny gets this from Ned.
Ned: And as for you, I don't know you but I'm sure you're a jerk!
Lenny: Hey, I've only been here a few minutes. What's going on?
On Ed, Edd n Eddy, Edd is always beaten up along with the others when Eddy's schemes go south even though he's usually the voice of reason, trying to convince the other two that they should be doing something productive instead. This was Lampshaded on one occasion: after beating up the Eds, Sarah admits that Edd didn't deserve the beating but then she adds "Give those cute ones an inch, and they'll take a mile!"
Much in a similar manner, Frizz and Nug of The Dreamstone are usually dragged kicking and screaming into each scheme by the Urpneys, usually via bullying from Sgt Blob and Urpgor or threat of death from Zordrak. The heroes have no problem punishing and humiliating them as much as they would their superiors, to the point they usually get the worst of it.
The Hey Arnold! episode where a drill sergeant replaced Mr. Simmons. He punished 3 kids by making them stand in room's corner. He makes Stinky do the same because of "symmetry".
Happens in American Dad!, when Steve is targeted with his other friends, who misled him into believing three of the Alpha Bitches sabotaged girlfriend Debbie's chances for election, leading to some Misplaced Retribution. Steve apologizes and reveals it was his friends, but he gets targeted along with them.
Ultimately inverted, as the friends posted bad things about Debbie, but it was Steve who carried out the (Brutal retribution) on the girls. The friends were at fault for tricking him, but everyone was mad at Steve because he did those horrible things.
In one teaser of an episode of Batman The Brave And The Bold, the sidekicks were forced to fight a training simulation as punishment for fighting each other. Speedy was one of them even though it was Robin and Aqualad who fought each other.
Happened at the end of the Fairly OddParents movie Channel Chasers when Timmy wished that everyone forgot the events of the movie. Crocker had nothing to do with the plot (in fact, he only appeared in the movie at the end), but he was still affected by this wish anyway. Which was a gag to throw in his brief moment of sanity from his FAIRY GOD PARENTS! obsession and reinforce the status quo.
Subverted on "Total Drama Island". Even though Heather is often helped in her schemes by another camper (usually Lindsay), all the blame is usually given to Heather. An example is when she causes problems between Gwen and Trent and everyone put the blame on Heather, even though Lindsay played an active part. Beth and Izzy never seem to get any hate either despite being members of Heather's alliance.
A frequent gag in Dan Vs.. Dan and Chris enter some establishment together. Dan proceeds to rant, make a scene, and piss people off, while Chris tries to stop him. This escalates until Dan and Chris get thrown out, and possibly get beaten up beforehand.
In one episode of The Jetsons, when Elroy brought home a bad report tape (that a failing student switched with his), George sent him to bed without dinner, but he also sent Astro to bed too, just because Astro believed Elroy wasn't lying about the tape being wrong. Boy, talk about strict.
Schools often employ this tactic to keep students under control in chaotic classrooms, much to the chagrin of well-behaved students. In more recent years it's become something of a Discredited Trope, as teachers have gradually come to realize that the badly-behaved students often enjoy getting their more well-behaved peers punished for no reason, and can actually cause the ones who normally behave to start being disruptive themselves once they realize they're going to be punished either way.
This can be used as a punishment in military training, and can and has been exaggerated to the point of only one person "eating the mousse". This can have the intent of banding the group together against an unfair superior or of having the group further punish their guilty peer and leaving the guilty party an outcast for at least as long as they continue to misbehave. Whichever result is intended can, of course, backfire into the other result.