One-Hand Jack
In one of the ending scenes, Cal handcuffs jack in the boiler after catching him and Rose doing the deed. But as disaster strikes, Rose decides that she needs to save Jack, so she uses an ax to break the handcuffs. It went just as well as you’d expect. By that, we mean it worked.
But while shooting the film, there’s a very clear shot of Kate swinging the ax and hitting Leo right in the hand. It was obviously a dummy ax, but if James wanted to maintain realism after this mistake, he would’ve gotten a new nickname: One-Handed Jack.