Firefly
After Firefly crashes into Applejack and has the basket stuck on her head. The other ponies laugh. Firefly is shown laughing too. But, later she is shown getting the basket pulled off of her head.

Mystery Pony

Maybe not a blooper, but an oddity none the less is the Mystery pony. She is the mulit-colored pony always shown with a yellow/orange cat on her back. She has sprinkles as her symbol. Maybe she is Confetti?