Mission: Impossible (1996)

Revealing mistake: When Ethan uses the magnetic screwdriver to undo the screws holding the grille to the air-vent in the NOC list room, the spindle that is actually turning the screw is visible after it falls out.

Revealing mistake: When we see a close-up of a knife being pulled out of a man's stomach, it's obviously a prop knife where the blade goes into the handle, as we can see the end of the blade while it's being pulled out.