Wires that are visible
Stuntmen have one of the most complicated and underappreciated tasks on set, as what they perform requires a great deal of bravery. Setting up the stunts necessitates being cautious not to damage the stuntman while also not revealing the stunt to the audience.
A man is dragged into a hole which has formed on the deck of the Titanic in one scene. And you can plainly see cables tied to the actor tugging him under throughout that sequence, especially if you slow down and look frame by frame. Doesn’t it shatter the illusion a little?