Revealing mistake: In the scene where Doc Brown is following Biff into the tunnel, as Marty starts to pass between the cars on his hoverboard, one of the wheels on the rig supporting the hovering DeLorean is visible. (01:30:30)
Continuity mistake: When Nick is in the spoon, the milk around him turns green.
Visible crew/equipment: During Bud and Coffey's fight in the lab, we see a lightbulb swinging from side to side between the two men. If you look closely, you can see a hand (belonging to a third person) pushing the lightbulb. The only people in the room are Bud and Coffey. (01:44:16)
Revealing mistake: When they gather around to watch what happens when you put the slime in the toaster, you can see when the toaster starts popping that it has little pneumatic pistons attached to the bottom, making it jump. When the toaster gets picked up off the table you can view its power cord which is entirely too big - more likely air lines. (00:41:15)
Continuity mistake: When Wallace races to get crackers, check the packet's logo. When the camera angle changes it's different. (00:08:57)
Continuity mistake: During the classroom scene, the spelling of of Genghis Khan's name on the chalkboard changes from "Genghis Khan" to "Ghenghis Khan" between shots. (00:04:50)