Revealing mistake: When we first see Max, his paws are leaning on a balustrade and become transparent. Though it may seem like it's an optical effect because they're in the shadows, the next angle shows them fully lit and opaque.
Revealing mistake: Near the end of the movie, when Wallace says: "hold tight lad, and think of Lancashire hotpot" one picture of a sheep on the ceiling slightly moves a few times. (00:19:57)