Continuity mistake: When Doc Brown lifts off in the Delorean just after Marty gets the Almanac from Biff, Doc does a full right turn on the steering wheel, but the next shot shows the Delorean turning left in the air.
Continuity mistake: At the beginning of the second movie when Marty is checking out the new car, he audibly bangs the car with his fist right below the window frame. In the version of the same scene at the end of the first movie, Marty makes a similar hand motion but does not bang the car, and there is no noise.
Continuity mistake: At the end of the preview for BTTF3, Marty and the Doc are standing in a position for their photo to be taken. The photo is taken, but on the photo their hands are in different positions to what they were when the photo was being taken.
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.
Continuity mistake: In Part 1, at the dance the singer sings "A fool in love with you", Lorraine and George don't kiss. In Part 2, the same scene is re-built, and now when the singer sings that, Lorraine and George do kiss.
Continuity mistake: In 'Part 1,' after Doc crashes into the trash can, Marty waves his hand in front of Doc's torso. In 'Part 2,' he waves his hand in front of Doc's glasses.
Continuity mistake: When Marty dries his jacket, the clock tower pillars are in the shade. But when he hands the hover board back to the girl, they are in the sun.
Continuity mistake: When Biff, in the alternative 1985, presents the record book to Marty, he puts it in a safe behind a portrait, and then he closes it. Biff continues speaking and you can see that the portrait is slowly opening again. Cut to Marty, then back to Biff, and you can see that now the portrait is completely closed.
Continuity mistake: In the Cafe 80's sequence we see little Elijah Wood and his friend investigating the Wild Gunman game. One of them asks "How do you play this Thing?" while Marty is several feet away, but Marty answers immediately while standing right next to it. So he has managed to walk about 15 feet or so in a fraction of a second.
Continuity mistake: In the tunnel chase where Biff is catching up with Marty, the speed of his car changes from going fast and gaining speed to driving at normal speed a couple of times in different shots. The distance between Marty and the car also keeps changing with this.
Continuity mistake: When Gramps is knocking Marty with his cane, right after he cries "Mr. Loooseer.", he puts his cane in his right hand, but a split second later it's seen on his left hand.
Continuity mistake: When the DeLorean first lands in the alley in 2015, there is a shot of the back of the car as it lands. The large vents that extend out of Doc's modifications to the car are the wrong size. In most shots, they extend over the car's tail lights, however in this shot they stop above the lights. According to commentary this is a fiberglass scale model of the DeLorean used to film flying scenes, since it is lighter than the actual vehicle, so apparently the model was not quite identical to the real DeLorean.
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: When Marvin Berry leaves the stage to call his cousin Chuck Berry, in Back to the Future II he stops and walks around a ladder that Mary climbed to get on the catwalk over the stage. Watch Back to the Future I and when he is talking on the phone there is no ladder behind him. In fact, that whole scene is different. The desk is different along with the phone and Marvin has his back to the stage in II whereas in I he is facing the stage. (01:26:09)
Continuity mistake: In the original BTTF the photo used to determine if Marty would exist in 1985 (the one with his brother and sister) is a wallet size photo. Yet when Marty in BTTF II (with the leather jacket) sees the other Marty from BTTF look at that photo and rush back to the dance, it's a regular size photo. (01:22:27)
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).
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.