Transformers: After figuring out one weakness of the alien robots Josh Dumall tells one of his troops to relay that everybody should load "sabot" rounds. Maybe I'm mistaken, but I thought sabot rounds were for shotguns and main-battle tanks? I've never heard of sabot rounds for an M4.
Terminator 2: The famous sceen of Linda Hamilton cocking the shotgun by holding the slide and flipping the shotgun up and then sharply down.
Preditor: The mini-gun scene. The whole scene was fantesy, but the concept that you could carry enough ammo in that dinky back-pack to unload for the 3 minute scene is laughable.
Die Hard: Willis dives across the room shooting 2 Beretta 92's (IIRC), in dramatic slow-mo of corse, and actually hits targets. Meanwhile the bad guys, with full auto weapondry can't hit the fully laid out Willis in mid flight.
Any show that depicts the hero using a snub-nose 2" barrel revolver to drill an escaping convict 187 yards away or down an aircraft.
The obligatory scene where maniac criminal mastermind jams a gun in a hostages face and yells his demands. Then, when the hostage bravely remains silent, the lunatic mad-man, who has likely killed several people by this point in the movie, pulls back the slide on his automatic to chamber a round and repeats his demands. Maybe it's not a mistake, per se, but it doesn't make sense that (1) a raving wild-eyed killer would carry a weapon without a round chambered (2) said killing machine wouldn't have a round chambered when threatening the hostage in the first place (3) that having fired his weapon earlier in the movie, this homicidal killer would then unchamber the round "just to be safe"