Continuity mistake: When Hagrid drags the Christmas tree into the castle, in the first shot there is no snow on his hair, although he has supposedly been outside for some time. When the camera angle changes his hair is now covered with snow. (01:25:10)
Visible crew/equipment: Hagrid surprises Harry with the box of birthday cake and Harry opens the box. In the close-up of the cake, as Harry holds the box with his left hand and moves the blue string with his right hand, a crew member's finger is visible under the box, over Harry's left shoulder. Obviously it is someone holding the box underneath, while Harry holds it with one hand. (00:14:10)
Visible crew/equipment: When Malfoy steps onto his broom while holding Neville's Remembrall, the actor's two curved chalk marks can be seen on the ground near Hermione's feet, and just as Harry steps backward we can see the actor's chalk marks on the ground near his feet. (Fullscreen version). (00:57:30)
Continuity mistake: On the train, when Ron introduces Harry to Scabbers there are two Jelly Slugs lying on Ron's leg - one red-green and one yellowish-brown, but when Ron casts the spell on Scabbers both Jelly Slugs are red-green, and then it switches back again. (00:36:40)
Visible crew/equipment: After entering the Key room Harry says, "They're not birds... I'll bet one of them fits that door," and as they reach the levitating broomstick, at the bottom right corner of the screen, the shadow of the overhead boom mic moves on the floor. (01:58:20)
Continuity mistake: During the scene with the troll, Hermione crawls over to where the sinks are and if you look at the floor you will see that there are only a few wet spots. In the very next shot, however, the whole floor is soaked (this occurs before the troll breaks all the sinks). (01:10:40)
Continuity mistake: When we first see Hermione on the train, her tie is crooked. When she sits down, as Ron is about to change Scabbers yellow, her tie is straight and it remains that way while she is seated. When she leaves and turns around to tell Ron about the dirt on his nose, it is crooked again. (00:36:05)
Continuity mistake: When Quirrel collapses in the great hall on Halloween, his feet are quite far apart. Camera cuts to everyone rushing for the door, and Quirrel's feet are practically touching. (01:05:30)
Visible crew/equipment: When Dumbledore gives the order to the students to follow the Prefects towards their dorms after the troll came out from the dungeon, just a moment before Harry warns Ron that Hermione is unaware of what is happening, some light reflectors simulating lightning are reflected on the glass above the pathway where the students come from.
Revealing mistake: When Ron is hit by the Queen during the chess game, the stunt double is visible on the horse just a few moments before he falls.
Chosen answer: There are several reasons. First, the position is cursed, so there is little reason to give Snape the job when he will not last any longer than all the previous instructors. If Snape did become the DADA instructor, something could happen to him that could result in his being harmed, having to leave Hogwarts, or be otherwise incapacitated in some way; that would render him useless to Dumbledore as a double agent. Also, Dumbledore trusts Snape, but putting him in an environment where he is teaching about dark magic on a daily basis would be too tempting and emotionally compromising to someone who had been so easily seduced by the dark side. He could possibly relapse. It would be like having a recovering alcoholic work as a bartender. Of course, he does eventually become the DADA instructor, and lasted no longer than his predecessors.
raywest