Revealing mistake: As the grating descends on Raoul, in two shots, large bubbles are flowing downwards. It's obvious, the film was reversed. (01:59:15)
Revealing mistake: During the second kiss, you can see behind Gerard Butler's neck (going from under his hair down to his back) the wire of the apparatus used to keep his lower right eyelid pulled down. (02:06:05)
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. (00:22:05)
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 This continues until he and Christine reach the top catwalk when the neatly trimmed sideburns have been replaced by pretend sideburns and rubber prosthetics necessary for the removal of the Phantom's mask.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. (00:46:15)
Answer: I like to think that he would always place a rose there, possibly every week or so, and they never had the ring on them. And he knew When Raoul would be there, so he placed them when he knew he would be alone with her. As for the two years later, since the phantom "knew" when Raoul came... he especially put that ring there. Just to let Raoul know about his love for Christine and the fact he is still living. He knew Raoul would visit then, and knew what to do. (This is only a theory I have proposed. As for a genuine answer... I am not aware of one.) Hope this maybe helped.