School Song - S4-E24
Continuity mistake: Zack messes with the piano so it will give him a chance of winning. When Tori plays it sounds like she is messing up. But immediately following that screech plays the piano with no problem and there was no time to fix the piano.
Continuity mistake: In the 1st season (the Ms. Bliss episodes), Zack mentioned to Screech in one episode that he had good news for Screech's brother. But in the Bayside episodes, Screech confirmed being an only child (specifically said in this episode).
Continuity mistake: When Tori joins the group during the final season, she and Zack date for two episodes, but they are not in sequential order.