Continuity mistake: After George knocks out Biff, Biff has an obvious contusion and blood on the right side of his nose and mouth. As the hoverboard chase scene progresses over the next several minutes (particularly in the lighted tunnel), the contusion and blood repeatedly disappear and reappear on Biff's face from one camera cut to the next.
Continuity mistake: At the end of BTTF 1, when Doc tells Marty and Jennifer that they have to come back with him to 2015, while they are sitting in the DeLorean, the Doc says," ...where we're going we don't need (pause) roads," and as he says "roads" he puts his silver glasses on normally. But in the beginning of BTTF 2 in the same scene, when he says," ...where we're going we don't need (pause) roads, he swings his glasses on really fast with only one swish as he says "roads."
Continuity mistake: When Marty is reading the letter from the Western Union man, he's standing with the letter in front of him at chest level. Switch to side camera angle and he is now bent over, holding the letter down to the car's headlight. Switch angle again and he's standing up again, with the letter back in front of him.
Continuity mistake: At the end of BTTF, when Doc drives up to Marty's driveway and tells Marty and Jennifer to get in the car to go to the future, Marty asks Doc, 'Why, what happens? Do we become a**holes or something?' Then Doc replies straight away 'No, you and Jennifer turn out fine, it's your kids, something's gotta be done about your kids.' At the start of BTTF2 when the same scene is played again, Marty asks the same question but this time Doc pauses before he replies to Marty's question. (00:02:00)
Continuity mistake: When Biff and Marty fighting over the almanac while driving towards the tunnel, the almanac flies out of their hands and lands on the front windshield. Biff slowly reaches over the windshield blocking the center rearview mirror entirely with his right shoulder. Cut camera angle and the mirror is unblocked, showing Marty to the rear of the car, then a reaction shot of Biff seeing Marty in the mirror, all while he is still blocking the mirror with his shoulder.
Continuity mistake: When Marty gets locked in Biff's garage, we see that he can't push through the two garage doors because there's a lock on the other side. Yet, when Biff comes out to go to the Prom dance, he opens them in literally no time at all with no resistance. We don't even hear him unlock them.
Continuity mistake: When Biff is trying to run down Marty on the hoverboard, as he comes out of the tunnel there is a shot of the manure truck driving down the road. Cut camera angle to Biff missing Marty and hitting the truck, which is now at a dead stop, however there was not adequate time to do this and appears that the truck was driving then instantly stopped.