Back to the Future Part II (1989)

Continuity mistake: Notice Marty's hair in the scene when he's playing "Johnny B. Goode." It's quite larger than when he says, some seconds later, "I guess that you're not ready for it yet." That's because when he says that, it's a recycled shot from the original BTTF, whereas most shots showing he's playing the guitar are newly made for BTTF2. However, Marty's hair is considerably longer in the new shots. (01:27:19)

Trivia: The reason that Marty McFly Jr. was arrested was because of stealing an unsubstantiated amount of money from the Hill Valley Payroll Substation.

Question: Doc Brown strongly believes that nobody should ever find out about their own future. With such a strong conviction, why would Doc tell Marty that his children going to prison is the one event that would ruin the whole McFly family?