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.
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.
Continuity mistake: During the movie, Max's shoes change, and they obviously can't because he's trapped in the hypercube. When helping the colonel who's hanging himself, you can see Max's shoes are black, but then later on they're blue. This is most noticeable in the scene when Max and Julia are talking, and Max throws his shoe across the room.
Continuity mistake: When Simon first takes Kate down, he covers her mouth with his hand, then with the other hand, holds a knife. In one shot he holds the knife to the side of her face, but in the next closeup shot, he has the knife resting on his other hand, which is covering her mouth. Then in the next shot he is holding the knife to the side of her face again.