Continuity mistake: In 1985A, when the punks fire at Strickland, there's a red flower in the pot closer to Marty. There's a first bang, angle changes and the flower has disappeared. If the bang had hit the flower Marty would've been injured, for he was behind it.
Continuity mistake: If you look at the destination keypad, you will see that the PM is over the AM. In the first "Back to the Future" film (1985), the AM is over the PM.
Continuity mistake: When Lorraine, Biff and Marty are in the hotel in the alternate present, after Biff has said "...and all three of them?", Lorraine puts her drink down on the table with her left hand. In the very next shot, she is holding the drink in the other hand.
Continuity mistake: In the first BTTF at the scene were George Mcfly knocks out Biff and takes Lorraine, Marty sees them while he is standing in front of a couple. When he goes through the couple to get to the band performing at the dance, the woman in the couple makes a noise and gets pushed aside as he tries to get through them. In BTTF 2 when the Marty back from 1985 is watching George Mcfly knock Biff out before he steals the Almanac, the Marty from 1955 stands in front of the couple again, but this time the couple leaves a gap for him to go through, and when he goes through the woman in the couple, she makes no noise and doesn't get pushed. It was as if she was expecting it that time. As well, the couple isn't the same.
Continuity mistake: In the high school dance scene, the guitar amplifier is standing on the floor. In the same scene in Back To The Future 1 it's raised up on some kind of box.
Continuity mistake: Biff's 46 Ford changes from a convertible to a hardtop with the roof cut off during some scenes (notice the shape of the windshield frame and vent windows).
Audio problem: As Marty and the sales guy are watching the sports flash thing, Marty makes a comment about the Cubs winning against Miami, and when the sales guy says, "Yeah really something isn't it? Who woulda thought," his lips don't match what he says.
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: 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: 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: 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: 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.
Factual error: In 1955, Doc says "Sunrise should be in around 22 minutes." This wouldn't be possible as the sky is fully dark (except for a slight bit of light on the other side). If only 22 min from sunrise, even on a stormy overcast morning, the sky would be fairly light by then.
Continuity mistake: At the end of the first film when Marty leaves the garage to greet Jennifer, he does not have a watch on throughout the whole scene. In the identical scene at the beginning of this film, Marty is wearing a black watch.
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.
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