Continuity mistake: At the end of the movie, before the Guardians leave, Jamie rushes over to hug Jack. In the first shot, the top of his head nearly reaches Jack's ribcage. However, in the following close-up shot, Jamie is almost at the same height as Jack's shoulders. (01:27:30)
Continuity mistake: North takes Jack for a tour of his toy factory. He tells one of the yetis to make the robot he's doing red, to which the furry beast wookie-groans frustrated, as there's already a pile of them he needs to scrap. Said pile next to him in the first shot looks full, but when it cuts to a wide shot you can see that the top rows are missing the leftmost robot (or rather, the rows start suddenly for no reason from a spot more to the right). At the very beginning of the tour, in fact, you could see the robot-making yeti in the background, and the pile was half the size we see in the dedicated sequence. (00:23:00)
Revealing mistake: During Jack's memories, when he is holding the antlers up to his head, Jack's sister's boot can be seen coming through the fabric of her dress.