My recently seen favorite, someone "cocking" the "hammer" of a Glock, sound effects, thumb sweep, and everything. With the camera perspective from the front so that it was not obvious. I expect this kind of thing from Hollywood, but I can only take so much...
I can top that. I once saw a scene in a movie (forget the name), in which the "tough as nails" lady detective chambered a round in her Beretta 92 by holding the gun in one hand -- by the slide -- and them sharply snaping her arm (snap-snap). The frame shot back and forth due to the momentum, and she had that satisfied "I know what I'm doing, I'm the only one in this room professional enough to chamber a round in a Beretta 92" look.
It was obviously a "Hollywood gun", rigged with a featherweight spring so that the slide would operate when firing blanks. Mz. Actress however didn't know that. She obviously knew that "the gun" was so easy to work, and so, improvized that "tuff move" (this was before Hollywood figured out that guns were a lot deadlier when held sideways) to show her stuff. That was "Idiot Move #1". "Idiot Move #2" was when the director said to himself, "I like it, I like it!" and kept it in the film.
Enough of that kind of crap and you start thinking that maybe they should require a proficiency test before letting someone handle a firearm. Or at least an IQ test. :)