Private detectives are always smarter and more resourceful than the police. They are more effective at solving crime even though they lack the ability to obtain search warrants, examine confidential police files, question witnesses under oath, and so on.

Any piano player can easily play and carry on a conversation at the same time. (My friend who plays piano calls this the "Dooley Wilson syndrome." DW played Sam in "Casablanca." She says people are always trying to talk to her while she plays.)

Computers and electronic control panels always have an array of tiny lights that flash in a rhythmic, repetitive pattern. (Real life: even when computers did have rows of tiny lights, they didn't flash in this way.)

Sound travels at the speed of light. Anyone who witnesses an explosion, cannon fire, etc., no matter how distant, sees and hears it at the same time. Lightning and thunder occur simultaneously as well.

Electricity (or the lack of it) travels slowly. When an explosion takes out the power plant, the city starts going dark after a couple of seconds delay. If the entire city is visible in an aerial shot, block after block can be seen going dark like a row of dominoes falling.

Space ships, space stations, and other futuristic environments always have sliding partition doors. Sometimes they even slide in a complicated way, opening up like an iris, for instance. (In real life, the technology for building sliding doors has been around for a long time. If sliding doors are clearly superior, as science fiction set designers have been telling us for years, why haven't all our doors been replaced by sliding doors by now?)

No one ever has any inkling that a husband/wife/boyfriend/girlfriend is cheating on her/him until she/he accidentally walks in on them in the bedroom, finding them naked and "in a heap." Adultery is never discovered any other way.

In a dark room, there will always be a small crack of light somewhere that will light up the actor's eyes.

(Say, hasn't there been another thread on this topic? I'd swear I've posted some of these before but I can't find them.)