Revealing mistake: In the first overhead shot as the Dementors swoop down towards Sirius and Harry, Sirius lies in a particular position. During the Dementor attack his clothes move, then after Harry's Patronus repels the Dementors, Sirius breathes again and he now lies flat. After Harry collapses, in the last overhead shot, Sirius and his clothes lie in precisely the same position as the first overhead shot. His position differs later, in the repeat timeline as well. (01:42:50 - 01:45:20)
Revealing mistake: When Harry and Hermione have gone back in time and they are running across the courtyard beneath the clock tower, you can tell this shot is using CGI because they seem to be running faster and further than the number of steps they are taking. (01:44:05)
Revealing mistake: The first time the bus conductor shows Harry the newspaper with the picture of Sirius Black on the front, one of his fingers disappears "underneath" the moving picture for a split-second.
Revealing mistake: When the students are running towards the portrait of the hippos where the Fat Lady is hiding, watch the railing of the stairs. It shakes back and forth, showing it isn't really made of stone.
Revealing mistake: During the Knight Bus scene, Harry smacks into the glass twice. Both times this happens, it's the same shot used twice.
Revealing mistake: In the punch scene "replay", Harry is noticeably stockier, and I would suggest shorter, and he walks in a very unusual way - very different from Harry. Obviously a double.
Revealing mistake: When Mr Weasley is warning Harry about Black wanting to kill him there is a jug in the background supposedly magical because it is serving the drinks by itself. If you look closely you can see the wire or string or whatever held it up.
Revealing mistake: When Hagrid stands up and nearly knocks over the table, McGonagall's goblet manages to stay rooted to the spot.
Chosen answer: Fred and George used the map to aid them in their mischievous activities at Hogwarts, and probably had little interest in what their little brother was doing or who he might be with at any given time.
raywest ★