The Legend of Sleepy Halliwell - S6-E14
Other mistake: In previous episodes it is stated that good witches don't freeze. But after Piper has been decapitated and her head is placed in the manor, one of the students freezes all the kids by tapping into Piper's freezing power. The kids are all good witches so they should not have frozen.
Other mistake: During Pacey's phone call to Joey at the end of the episode, Pacey lies about being on a fishing trip and mentions having caught a "91 pound marlin" which is a pretty big fish. This doesn't make sense for him to say that, as a fish that large would be noticably missing when he and Doug returned home. And they wouldn't have likely thrown a fish that large back into the sea. It would reveal he was lying if he returned home without the fish.
Other mistake: During the first dozen or so episodes, during the opening credits, Onizuka is shown spray-painting a picture of a woman on a wall, with the words "Love" and "Piece" written next to her. Obviously, "piece" is a mistranslation, and the wrong spelling. The correct text should read "Love" and "PEACE."