Revealing mistake: In a scene soon after "Think of Me," it shows a large crowd of actors, crew, etc. celebrating backstage. In one far-away shot, you can pick out Andre and Firmin in the center flirting with two of the women. Andre sticks his cigar in a woman's mouth, and she blows a puff of smoke. A few shots later, it shows the same thing, but much closer up and from the side.
Revealing mistake: When the Phantom first appears in "Don Juan," he is wearing his shorter full face mask showing his neatly trimmed sideburns. This continues until he and Christine reach the top catwalk when the neatly trimmed sideburns have been replaced by pretend sideburns and rubber prosthetics necessary for the removal of the Phantom's mask. The prosthetics also shine differently in the stage lights whilst the Phantom is wearing them. Most obviously as he arrives at the top of the stairs of the catwalk.
Revealing mistake: The second time we see the Christine mannequin, we can see it sway from right to left. The swaying can't be due to a draft, since any sort of wind would have made the candles flicker. The moving is due to the fact that Emmy Rossum is the mannequin. They couldn't get a wax figure to so closely resemble her, so Emmy sat very very still (although not still enough!) and played the mannequin herself.