Towards the end when the Phantom's lair is collapsing, you can see bricks falling into the sewer water and floating. Obviously they were made out of light-weight material and not out of actual stone.
When Claudin thinks his concerto has been stolen, he goes into a frenzy and attacks the music publisher. The publisher's assistant, Georgette, stops the attack by splashing a tray full of powerful etching acid into Claudin's face. Oddly, Claudin does not recoil when she throws the acid; rather, he leans forward, as if to ensure that it the liquid strikes him full in the face. Which is precisely what happened. To avoid having to shoot the messy scene more than once, we see actor Claude Rains deliberately leaning into the splash for a thorough dousing.