Revealing mistake: The two unicorns are a stallion and a mare in the movie. But both horses used were clearly stallions. This can be seen quite well when the "mare" is jumping around after she comes upon the stallion with his horn removed; a full shot of the male horse anatomy.
Revealing mistake: In the opening scene where you see the bear walking by, you next see a deer standing. The deer has a rope around its neck and is tied to something.
Revealing mistake: When Blix fires a fireball at the goblin in the woods with the unicorn horn, the flame sets fire to the goblin's behind; he sits in a bucket of water to put it out but the flame goes out before he actually sits right down or the 'hiss' of steam appears.
Revealing mistake: After Lily runs into Nell's house to escape the goblins, the baby in the crib is a doll.