OK, Maybe not a slip up and maybe so. I know it's just a movie, but I noticed that when the students enter the Great Hall/Dinning room, the doors open inward. Later on when Prof. Quirrel comes running in yelling about the troll, the doors also open in (will watch when I see the rest of the movies again). Now for fire & other safety reasons, all inside doors should open out to prevent people from getting trapped inside from being unable to open the doors.

Maybe because they are wizards & witches, they can just use a wand and make them open anyway needed?