Character mistake: When Buquet hangs over the scene, you can see Meg screaming along with three other dancers. Four shots later, Meg is still screaming, but the two girls who were to her right (on the left of the screen) are still dancing, as if they haven't seen Buquet yet. (01:05:10)
Continuity mistake: As Christine and Raoul race up the stairs of the Opera House, her hair is loose. By the time they come out onto the rooftop, it's partially pulled back in an elaborate fashion. She's never off screen long enough to have done that. (01:05:35 - 01:07:05)
Audio problem: Right after Joseph Buquet is killed, Raoul runs and finds Christine. When she says, "Raoul, we're not safe here", her lips do not move; her mouth is merely agape. (01:05:35)
Other mistake: While running to the rooftop, Christine always leads Raoul. She is the first to get up the stairs, and there's no room for him to pass her without her stopping her frantic pace (which she shows no sign of doing), yet somehow ends up only rushing out the door after Raoul does. (01:06:30)
Continuity mistake: On the Opera House's rooftop, the lining of the Phantom's cape goes from light-coloured to black before and after he jumps on the statue. It has nothing to do with lighting, and his waistshirt proves it: much darker than the cape's original lining, it makes it obvious the cape's turned dark since the waistshirt looks paler against it. (01:15:55)
Deliberate mistake: On the rooftop scene after Raoul and Christine have left the Phantom, he bends down to pick up a rose, then runs to the statue and climbs up, and finishes his song. There are footprints in the snow all the way to the base of the statue. The footprints are a definite error, as they are next to each other instead of spaced apart which would have occurred if a person was running. And the footprints were not made by Christine and Raoul, they are in the wrong place for where they were standing. (01:16:10)
Audio problem: Several times during "Masquerade", starting with the very first shot of the chorus singing, the audio does not at all match their lip synchs. (01:18:10)
Continuity mistake: During the Masquerade, there is a close-up of two of the dancers, who sing "But who can name the face?" while standing next to each other, without moving. Next shot, they are several feet apart and the lady is dancing towards where the male dancer is standing. (01:18:50)
Deliberate mistake: When the Phantom throws his score for 'Don Juan Triumphant' down the stairs, the portfolio only reads 'Don Juan.' (01:23:20)
Visible crew/equipment: Just before Raoul jumps down the hole after the Masquerade, Patrick Wilson is obviously replaced by a stuntman who's not even trying to hide his face. He is the same stuntman used for the Phantom in the graveyard swordfight. (01:25:35)
Continuity mistake: In the graveyard, when Christine walks down the main alley, her arms are hanging by her sides, but in the next shot, she is holding her bouquet in front of her hips. (01:36:40)
Continuity mistake: When Christine approaches her family mausoleum, behind her you can see an above-ground tomb that has chains around the top cover. There is something on top of the cover that looks like a bouquet of flowers covered in snow, and there is not a lot of snow on the tomb. Later, when Raoul and the Phantom are fighting and the Phantom slashes Raoul's arm. Raoul falls backward onto this same tomb, and when we see him from above, rising, the chains on his side are missing, and there is suddenly lots more snow all over the tomb. The bouqet (or whatever it is) is missing. However, in the next shot that shows the same tomb, now there is much less snow, the bouquet is back, and one-half of the side chains have reappeared. You can see the continuity by the tree at the head of the tomb, and the triangular wreath laid at the head of the tomb. (01:39:05 - 01:40:45)
Visible crew/equipment: During the graveyard swordfight, the Phantom's stuntman sometimes visibly replaces Gerard Butler. He has thin and very arched eyebrows and the mask doesn't fit properly around the nose. He is the same stuntman used when Raoul jumps down the hole after the Masquerade. (01:40:45)
Continuity mistake: At the end of the first part of Don Juan Triumphant, Carlotta throws her right arm towards her back right, holding a fan. In the next shot, her right arm is at her side and her left arm is pointing forward to her left. (01:47:20)
Audio problem: In the opening song of Don Juan, Carlotta obviously cuts early, although her voice is still heard hitting the high (and obnoxious) note. (01:47:20)
Continuity mistake: When Christine reaches the centre of the stage during her first appearance in Don Juan Triumphant, her arms are at her sides and she holds the basket in her right hand. The next shot, she is holding the basket in front of her, with both hands. (01:48:40)
Continuity mistake: The Moment Raoul falls through the trapdoor in the stairs and into the water, you can see the grid (that's coming down later) already in the water. For the grid to come down and drown Raoul, he must have fallen through it. (01:53:35)
Continuity mistake: When Christine takes off the Phantom's mask during Don Juan, the appearance of his face suddenly gets drastically CG enhanced. (This occurs AFTER Christine has already removed the mask, as the two screenshots show.) (01:55:10)
Continuity mistake: The aspect of the Phantom's deformity undergoes a major change between when Christine takes his mask off on stage and the later parts of the movie. (01:55:25 - 02:00:00)
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)
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.