Continuity mistake: In the shots facing Harry, when he unbuckles the belt around the Monster Book of Monsters and when he lifts the book to look at it, the wood box on the dresser near the the water pitcher faces the fireplace. In the rest of the shots it faces the bed. (00:14:30)
Continuity mistake: When Lupin is turning into a werewolf and Sirius comes at him while saying, "Remus, my old friend. Have you taken your potion tonight?" Lupin's shadow on the ground change position in the next shot. (00:35:20)
Continuity mistake: Harry moves closer to Snape at the Shrieking Shack, as he points his wand at the professor. In the first two shots Snape stands closer to the door, yet in the close-up as the spell hits he is farther from the doorway, so he can easily fly backwards for the stunt. (01:34:00)
Continuity mistake: When Harry and Hermione are back in time, Lupin's transformation happens a lot faster than the first time. Also, the first time, Hermione shouts "Harry!", then Sirius yells "Run! Run!", then Lupin howls. The second time it goes "Harry!", then howling, then "Run! Run!" (01:39:45 - 01:57:40)
Continuity mistake: When the Dursleys are in the dining room, the chair located near the telephone, on the kitchen counter, changes position throughout the scene. In some shots its back is right beside the phone, in others not, and then in other shots its back leans right up against the wall and in others it is positioned well away from the wall. (00:02:45)
Continuity mistake: At Snape's class when Hermione shows up and Ron says "Did you see her come in?", you see Potter turning around facing Hermione. When it cuts to Snape saying "Now, which one of you can tell me the difference..." you can see Potter is now looking at his book. When it cuts back Potter is facing in Hermione's direction again. (00:50:20)
Continuity mistake: At the end of the film when Harry gets on the Firebolt, everyone is running outside, and there is no crowd properly formed beside the water fountain, yet when Harry takes off there is a very large crowd watching him all of a sudden. (02:04:05)
Continuity mistake: When Aunt Marge is about to fly out the door, we can see Uncle Vernon leaning on the table with his back against the door and the dog biting his left foot. When it cuts to Aunt Marge coming out the door, we see Uncle Vernon facing the door and the dog is biting his right foot. (00:05:10)
Continuity mistake: When the train suddenly stops, Harry gets up and slides the door open. He pulls the door with a tight grip on the doorhandle, then he releases his grip from the handle and rests his palm on the doorhandle. But in the next shot he is gripping the doorhandle again. (00:19:25)
Continuity mistake: As the Monster Book of Monsters falls to the floor and chases Harry's feet, the ripped seat cloth of the chair Hedwig sits on is visible. This ripped seat cloth changes significantly a total of four times in the following shots. (00:15:15)
Continuity mistake: After Potter has opened the 'Monster Book of Monsters' he stumbles backwards and climbs onto the bed. When it cuts we see him climbing up again. (00:14:45)
Continuity mistake: When Marge flies away, she is pulling the tablecloth off. At the bottom of the frame are candles and a green bottle. In the very next shot both of these are gone. (00:05:20)
Continuity mistake: During the scene in which Uncle Vernon's sister Marge is turned into a balloon we see her floating out of the conservatory. A second before she is blown out the door we see from the shot looking into the conservatory that Aunt Petunia is to the right of the conservatory doors inside the dining room. When the shot changes, she is now standing in the doorway to the conservatory.
Continuity mistake: When Potter is leaning over the bed watching the 'Monster Book of Monsters' on the floor when it goes back under the bed, we can see the sheets on the edge are fine. When it cuts they are crinkled. (00:15:05)
Continuity mistake: When Neville magically makes the Snape Boggart wear his grandmothers clothes, in one shot the Snape Boggart clutches his red handbag with both hands by his chest, yet in the next shot he is holding the handbag down by his right side. (00:40:55)
Continuity mistake: When Harry goes back to Hogwarts and they stand before the Fat Lady portrait she refuses to let them in because she's trying to show them she can break a glass with her voice. When she cannot she smashes it upon a brick wall behind her right shoulder. Later in the movie after the students gather around the Fat Lady's painting after it's slashed the wall is missing.
Continuity mistake: At the Leaky Cauldron, while Harry sits at the table speaking with Fudge, in two shots facing Harry the empty soup bowl is right beside the salt shaker, but in the rest of the shots there is a greater space between the two. (00:14:20)
Continuity mistake: When Trelawney asks Ron to look in Harry's cup and read the tea leaves, the Grim within the cup is visible. When Harry looks in the cup the Grim is different, for example, it has an open mouth in this shot. (00:30:20)
Continuity mistake: When Harry is talking to Sirius (the first time), Hermione yells to Harry to get his attention about Lupin. In the background, you can see Hermione sitting next to Ron right as she starts to say "Harry." In the next shot, she is standing up farther away from Ron (this is visible in the widescreen DVD only).
Chosen answer: It's not known what it is. It is left to the audience's imagination of what exactly is in the room or what some witch or wizard was doing in there. This is only meant to be a humorous bit and is unrelated to the plot. It didn't happen in the book.
raywest ★