Deliberate mistake: Season 2, episode 10: After Matt is supposed to be dead underwater, you can see air bubbles coming out from his nose in a close-up, and in a far away shot his head is above the water so the actor can breathe.
Deliberate mistake: When Fluttershy is telling off the cockatrice, her back legs and her wings turn to stone. But when she gives it the stare and the camera is right in her face her wings are outstretched.