My pet peeve in Hollywood gunfights is cocking a gun. First, whoever has the gun waits until they are actually confronting someone else before they cock the gun. Second, they always seem to cock the gun multiple times, for dramatic effect. Just do it once, it is ready to fire from that point on.
Sometimes it gets really ridiculous. In *Demolition Man*, at one point Sly Stallone racks the slide on his semi-auto, and you can see the already-chambered round fly out.
Especially when they have been moving through the house for 2 or 3 minutes already, clearing it with all that high tension pointing of the shotgun, and then when they finally see the monster or the bad guy, they chamber a round, and you just go 'wait a minute, during all that tension filled dramatic searching through the rooms and up the stair case where only your supercool quick reflexes could save you, your chamber was empty?'