If a sitcom is around long enough, it will sooner or later do an incredibly lame parody of A Christmas Carol.
Also, Rashomon.
All female professionals–forensic scientists, computer experts, doctors, lawyers, detectives, whatever–are devastatingly beautiful and unmarried.
Also: if a sitcom is around long enough, the female lead will get pregnant. This is always the shark-jumping moment.
And “It’s A Wonderful Life”
On CSI:Miami is the devastingly beautiful medical examiner played by Khandi Alexander married?
Forensic psychologists are ten a penny, female, devastatingly beautiful, unmarried, and live alone in semi-darkened apartments with easy stalking access for deranged killers.
Yes, she is.
When detectives return to a crime scene that has already been processed in order to “go over it one more time,” they will find the most crucial piece of evidence in the second or third place they look. It will probably be hidden in the false bottom of a music box.
On any show, the person who has never fired a pistol will be the one to make the instantly-fatal shot of the bad guy with a detective’s dropped .38 with a 2" barrel. Off-hand. In the dark. From 25’ away.
The funny thing is that anyone who knows anything about New York address schemes would be able to tell this by inspection. Especially a cop. I can only suspect that the LA-based writers think that NY addresses have five figures like California ones.
(I used to be a messenger in Manhattan, so I’m a professional about addresses. 
)
99% of the time, there will be no reference to anything that happened in a previous episode, even if it involved something as emotionally traumatic as the death of a loved one.
People are willing to do anything for revenge, including blowing a multi-million dollar deal they’ve been working on for months, just to get back at someone for a minor slight.
Put up and bought on Christmas Eve - thus making idiot Dad search high and low for the last one in town. (Stupid sellers, never buying enough.)
An old cliche is that any underling character will tell his mother that he is the boss/owner of the establishment, making real boss fake being underling when said mother comes to visit. Hilarity theoretically ensues. Example: Lurch did this in the very worst episode of The Addams Family.
Yep, this is what I was going to mention: On TV, it is cold and snowy at Christimas time, but never at any other time.
Or, more generally, there’s never any weather on TV unless it’s crucial to the plot and/or mood.
Some others:
Character A tries to tell Character B something important, but is brushed off with an impatient, “Not now! Can’t you see I’m busy?” This is followed by some disaster that could have been averted had Character B only listened to Character A.
At least 60% of all weddings are halted at the last minute, when some chance circumstance or revelation shows the character involved how ill-advised the marriage really is.
Every TV show has at least one character who, in real life, would be too stupid to live.
The only time anyone on TV ever prays is when they are in a jam and are trying to make a bargain with God. These prayers are always answered, though in a way that could be interpreted as coincidence.
I can testify that the Burger King at the Presidio had a picture-perfect view of the GGB out its window.
That’s not how I remember it!
This recent thread on sitcom syndromes is most instructive.
That used to be the case - but many, if not most, shows have kept story threads going thru many episodes
I have a very specific gripe but it has probably occured in other shows as well. In season one of Friends it is established that Ross and Monica are siblings, and that Ross is Jewish. So why isn’t Monica? Also Rachel is Jewish as well. Why were there never any Hanukkah menorahs visible? Why was Monica always decorating a Christmas Tree? The only mentions of Hanukkah was Phoebe’s holiday song, and the episode with the Hanukkah Aardvaark
Don’t forget a body-swapping “Freaky Friday” ripoff, if at all possible (even “The X-Files” stooped to that). And a “Tootsie” ripoff where someone has to pose as someone of the opposite sex.
Characters will have apartments they could never afford in real life.
No one ever has trouble finding a parking space, and the open space is always right in front of wherever they have to go.
Not only does everyone have a goofy neighbor, but the goofy neighbor–hell, all the neighbors–barge in and out of each others’ homes without knocking at all hours. Not only does no one ever lock their doors, no one ever catches their neighbor in the shower, on the john, asleep, or naked–unless it’s necessary for the plot.
Even working class people never say “fuck” in their own home.
In courtroom shows: If the judge isn’t the star, he or she is corrupt, crazy, brain dead, or a martinet. Often in some combination.
Oppressed, downtrodden, and/or handicapped people never turn out to just be jerks in their own right.
The townsfolk never learn their lesson.