Continuity mistake: Enjolras is holding a book during his protest which disappears and reappears at will.
Continuity mistake: In the scene where Drew and his band are setting up on stage, just about to do "Heaven", you can see the Mic has the decorations on it. Then just before Lonny goes to put them on they magically disappear.
Factual error: In the scene where Dave is playing cards with soldiers, some of the paper money being gambled is a design of U.S. money not introduced until about 30 years later.
Other mistake: When Christine is shot near the end of the film, there's no blood on her dress or the Phantom's hands.