Continuity mistake: The garage where Marty's truck is parked is very different in the opening scene of BTTF II than in BTTF I. The gardening tools on the wall to the left were different, and the brown wall siding in the back of the garage was not there in Part I. (00:00:38)
Continuity mistake: When Marty is hanging onto the side of Biff's car as the sports scores are being said on the radio, Biff picks up the almanac to check them. You see him put it down in one shot, but when the shot switches to Marty's point of view, he puts it down again. (01:31:25)
Continuity mistake: During the dance hall scene, the first half of the guitar solo Marty plays (before the Eddie Van Halen section) is completely different to the one he plays in the original film. Whether this is just to give time for the jump sequence to take place or not is open to question, but all the same, although not many people would notice, it is a pretty obvious mistake. (01:26:00)
Continuity mistake: Near the end, when Marty is holding on to Biff's car and is trying to get the almanac, the direction of the almanac's cover keeps changing from shot to shot.
Continuity mistake: At the start of this film (and the end of the prior film), Marty and Jennifer talk with Doc who has just returned from the future to retrieve them. A shot looks over the rear half of the DeLorean at Marty and Jenn and we see the street behind them. Ignoring that Jennifer is now played by a different actress, there is a tree missing behind her, as is the truck with a camper-shell that was two driveways back. Parked in the street is a VW bus where a Pontiac Firebird was. Marty has also gained a wristwatch not present in the same scene in the earlier film. (00:01:40)
Continuity mistake: As the DeLorean lands in 2015 at the start, the car has silver center caps (between the wheel nuts) in the front and back wheels, but in the very next shot, they have changed back into the original black center caps. (00:06:00)
Continuity mistake: After the roof scene in alternate 1985, the time circuits show the Nov. 12, 1955 readout before Doc enters it. (You can hear him still pressing the buttons afterwards). (00:59:00)
Continuity mistake: When Marty gets out of the water after jumping from the hoover board, the board is a bit far from him. In the next shot it is right next to him, to help him pick it up and leave.
Continuity mistake: At the start when the DeLorean crashes into the line of six bins on the driveway one of the bins is half open. It is one from the right. In the next shot when Dr. Brown gets out the bin that is half open is two from the right. Then a bit later when the DeLorean comes out of the driveway all the bin lids are closed. The bin lids were never closed all through the scene.
Continuity mistake: When Marty is being chased by Griff's hover board gang, Marty grabs rope from a car. Just after, he has to avoid an oncoming car. One second he's holding the rope, the very next shot as he goes over the bonnet, the rope's gone. (00:19:38)
Continuity mistake: In the first movie at the dance, he plays the guitar quite horribly before his parents kiss. In this movie, there is a shot before they kiss and Marty is playing the guitar perfectly.
Continuity mistake: When Doc tells Marty, "Don't do anything, don't interact with anyone, and try not to look at anything," note Doc's red shirt design. Then it cuts to a shot of Marty as he says, "I don't get it, I thought you said this had something to do with my kids." The next shot facing Doc, as he says, "Look what happens to your son," his red shirt's design has changed, though it's the same style shirt. The same thing occurs with Doc's yellow shirt with the train design, but it's more subtle. (00:09:25)
Continuity mistake: When Old Biff drives into the garage with Young Biff in the passenger seat, Young Biff (body double) is not wearing a belt, but the belt reappears when they exit the car and Old Biff places the Sports Almanac in Young Biff's back pocket. Also, the Levi's small red tab is now missing from that pocket. (01:07:55 - 01:10:05)
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 start of the movie Biff is waxing George's car in the driveway with his truck parked next to it. However it completely disappears and isn't seen again even though Doc appears and parks where Biff's truck was, and Biff doesn't leave the premises.
Continuity mistake: Marty's new Toyota truck in the garage at the end of Back to the Future and beginning of Back to the Future Part II has different front license plates. In the first movie it's 2EZP916 then it's changed to 2BAK860.
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." (00:02:23)
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: When Marty's on the hover board on the lake, watch the Clock tower's entrance behind him: Depending on if it's a wide, medium or side shot, the number of guards change from one to two, or one that runs away but suddenly appears back in place.
Continuity mistake: When Marty is playing his song on stage (during the part where he knocks over the speaker) the music playing is at a different stage to what it was in the first film. The music is now a few seconds ahead.
Answer: At the end of the first film, Doc says, "what the hell" in response to the letter Marty gave him. So this shows it is not that strong a conviction, especially in the face of definite knowledge about something bad happening. It is also a way for Doc to repay Marty.
MasterOfAll