Continuity mistake: Near the start, Percy says "Boys upstairs and down on your left, girls the same on your right". However, after the three heroes have been to investigate Fluffy and they go back to the dorms, when they reach the top of the stairs Hermione goes to the left and the boys head right. (00:49:30 - 01:03:30)
Other mistake: While Harry and his classmates are on their way up to Gryffindor Tower right after the Sorting, Neville reacts with astonishment to a portrait moving. However, we learn in later films that Neville was raised in a magical household (by his grandmother), so he should have been completely accustomed to seeing pictures move.