- The sleepwalker who is dangerous to wake, and should not be disturbed by any means, even if in danger.
- A character gets fired and must find a new job. Always gets hired back eventually, maybe with a raise, promotion, pay cut, or demotion that is soon forgotten.
- A character's friends try to remove his bad tooth through methods not recommended by modern dentistry. A piece of string tied to the tooth and a doorknob is always attempted.
- Stolen bank money is found by, or on, one of the characters, and instead of turning it in right away, he alternates between agonizing over it, fantasizing about what to do with the cash, and/or being harassed by the real thieves.
- A new tenant in a character's house or neighbor makes everyone's life hell. Sometimes this tenant sticks around and overshadows everyone else, resulting in the show's refocusing on him. (I propose this be known as Urkelitis, or Cousin Ernie Syndrome.)
- A character moonlights as a department store Santa Claus.
- Characters appear on a game show, but do not win. The show is often one that actually aired at the time of taping, usually hopelessly dating the episode.
- A "real world” television or movie personality mentioned and obsessed over suddenly arrives in town to meet the characters, usually in the episode of mention. All such celebrities are nice, sane everymen who will either indulge a character in some act of lunacy, or call for the police when they display the typical situation comedy disregard for personal boundaries.
Some of these clichés exist because, to be emotionally involving for the viewer, some fundamental element of the characters' life needs to be in danger, but cannot actually be changed permanently because of the risk of breaking the carefully-formulated, situation-generating premise. So something big and important happens at the beginning of the story, but is resolved before the credits. In conversations about sitcom writing, I refer to this by saying that the events in question "cancel out of the plot." (In novels or movies, characters, situations and events which cancel out usually should have been removed. In sitcoms, most episodes cancel out because otherwise the premise would change.)
Because of this, fundamental changes in the lives of characters happen, for good or ill, at most, very rarely. Also, if some important event were to happen in a particular episode that would change the premise, it might break the continuity should that episode be shown out of order. Because of this, important episodes usually occur at the beginning of the series (setting up the premise), at the beginning of a season (because of actor death or firing, or major retooling during hiatus), at the end of a season (Can you say cliffhanger? How about two-parter?) or at the end of the series (Goodbye, Farewell, and Amen).
Because of the delicate web of tension (or various sorts) strung between the characters in a sitcom, the balance of power between characters does not change much over the course of a series. This is something to think about should you ever realize how much roleplaying games resemble situation comedies.