When André, Firmin, and Raoul are supposedly entering before "Il Muto", André, Firmin and the former manager enter instead. It's a reused sequence from the "Hannibal" rehearsals, which did not cut away before Monsieur Lefèvre (Simon Green) appears in the shot.
Ramin Karimloo's tattoo (on his wrist) is visible in one of the close-ups during "Music of the Night".
The overlay of Christine during the first verse of "The Phantom of the Opera" is actually a close-up of a later shot from the fourth verse. If you look closely enough as the shot starts to fade out, you can see the pole being lowered by the Phantom.
During "All I Ask of You" the Eiffel Tower can be seen in the background. The musical takes place in 1881, the Eiffel Tower was not constructed until 1889.
When Christine wakes and takes the Phantom's mask off, her mouth moves as if she's screaming but you can't hear anything. She's also heard yelping as she falls with the mask, but her mouth doesn't move.
Phantom scoring music with quill in right hand as Christine arises from slumber in the lair and later signing Opera Ghost letters with left hand.