Revealing mistake: When Hans discovers the first bad guy lying dead in the lift ("Now I have a gun - HO HO HO!") and pushes his head, the corpse is moving his head before he is hit (watch carefully). He also closes his eyes before Hans makes contact. (00:39:15)
Revealing mistake: One of the reindeer's "power-up" lights turns on before its name is called.
Revealing mistake: Near the end, when Eliot enters Frank's office with his rifle, he accidentally shoots into the ceiling. He shoots it at a slight angle, but the rubble falls right on top of his head instead.