Corrected entry: At the end of the movie, when the train is leaving Hogwarts, where is the big lake that they crossed in boats in the beginning of the movie? If you look at Hogwarts there is clearly no lake around it.
Corrected entry: When the Slytherins and Gryffindors have their first flying lesson, there is a girl wearing another house's uniform. <
Correction: It isn't specified in the movie that only Slytherins and Gryffindors are in this class. This is more of a movie/book discrepancy.
Corrected entry: In the scene in the Great Hall where Ron is looking at his Chocolate Frogs cards, Harry picks one up. It's suppose to be Dumbledore's card, but in several shots you see where Dumbledore's picture should be. Instead, all you see is the green screen attached to the cards.
Correction: Dumbledore's picture was a simple holographic foil image; the same kind you'd see on any credit card. There was no green screen effect needed for this. What you're seeing is just the way the light plays off the foil when viewed from an extreme angle.
Correction: There's no lake around the castle - it's on the south side. First years just cross the lake because it's tradition.