The Elements of Harmony (2) - S1-E2
Other mistake: At the end of the episode when Princess Celestia is talking to Twilight Sparkle you can see that Princess Celestia's tail is on top of her leg instead of in-between. It's covering her cutie mark.
Speeding Is Dangerous - S1-E22
Other mistake: When Shine says, "I think I'm OK," his front tire is layered over the railway tracks.