Visible Wires
Stuntmen have one of the most intricate and underrated jobs on a film set, as what they do takes a lot of guts. Setting up the stunts means you have to be pretty careful not to cause the stuntman any harm, but also not to make the stunt obvious to the viewers. In one scene in Titanic, a man is pulled into a hole that formed in the deck.
And during that scene – especially if you slow down and watch it frame by frame – you can clearly see wires attached to the actor, pulling him under. Kind of ruins the illusion a little, doesn’t it?