Best sci-fi movie mistakes of 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.
Revealing mistake: In the scene where Amy lets Wayne know that breakfast is ready, she presses a button labeled breakfast. There are other items on the box, such as lunch, dinner, and phone. When they show the board flashing the word breakfast, you can see that the word breakfast is the only word the board can display between flashes.