Continuity mistake: During 8th grade in one episode, Zack talks to Ms. Bliss about having had to use his sister's bike for a project. In high school, many episodes have Zack being an only child (further proven several times when Derrick Morris would tell Zack that "he and his mother were more important than his job").
Factual error: It seemed like they had several senior years in the show? They had the prom year when Kelly, Jesse, were there. Then they had senior year when Tori was on the show when they did the senior class rings.