Revealing mistake: When Kate opens one of the doors to find Simon on the other side ("Hello gorgeous, miss me? Huh? Come on in, the water's fine."), the shot of her climbing up to open the door is a recycled shot from 42 seconds earlier. It's pretty obvious as the shot still contains a mistake from when the shot was used previously - a boom mic is reflected in the door's shiny surface as Kate climbs up to open it.
Revealing mistake: When Kate decides to ditch Maguire and escape herself, we get a shot from above showing her face and Maguire's feet. However we only see his trousers. His feet are gone. Even though some could say that the wall removed them, but we can also see that the wall hasn't reached the part of the feet that are missing. It appears the CGI team forgot to add his feet in.
Visible crew/equipment: When we see first Kate open a door, it opens, and she leaves her hand up, then slowly lowers it. Look to the left of the cube-shaped doorway, and you can see in the room behind her, the edge of a stand or table or some equipment, which obviously shouldn't be seen there.
Other mistake: This occurs during the scene where the group has just rescued the colonel. Before Mrs. Paley makes her appearance, the others notice the panel opening, and make movements, but they don't make sense. We'll call the side where the colonel was the north. Sasha is in the southwest corner, and Max is near her. Simon and Jerry are both at the north wall. When the sound of the door opening is heard, Max looks to the west. Simon is kind of in a strange position, but looks correctly to the north, where Mrs. Paley comes from. Simon and Jerry both knew where to look before the door opened, so it's reasonable to assume that it's easy to tell which door is opening, so Max shouldn't have looked at any other door.