Revealing mistake: During the filming of 'Hell's Angels' Howard gets the idea to build a single wing mono plane. He and another take an axe and smash the support holding up the upper wing. If you notice the upper wing is already broken, hinged and a very poor covering has been laid over the joint. It is very obvious and noticeable in a few shots leading up to this.
Continuity mistake: As Barrie (Depp) and Frohman (Hoffman) are sitting in the theatre shortly after Barrie seeing all the sets being removed from the theatre, there are a number of guys removing what look like set doors. The first two guys that walk behind Barrie make a large distance between seeing them and walking down the hall. A few seconds later two more guys just walk past the back of Barrie and in the very next shot they have disappeared. (00:15:40)
Continuity mistake: When Chris is hitting the slot machine in the Casino, we see that the cover on the machine comes off, then the lights are off. But when the shot changes, the lights are on. (00:38:15)