Pilot (aka Emotions in Motion) - S1-E1
Deliberate mistake: Throughout the first episode, Jen's bangs are long in scenes relating to her "Grams" and short in all other scenes. This is because the scenes involving her storyline with Grams was shot much later, after the show was picked up. (00:05:45 - 00:16:15)
Wrestling with Demons - S3-E12
Deliberate mistake: When Piper froze the demons (near the wrestling) one of the demons moved when she froze him.