Continuity mistake: At the end of Back To The Future, when it shows Marty and Jennifer talking to Doc, Marty is tanned, but at the start of this movie when the scene is repeated, he is pale.
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.
Other mistake: In the scene where Biff confronts Marty outside the swinging doors while Marty is watching himself talk to his 1955 Mom and Dad, you can see that Crispin Glover (George McFly) has been replaced with Jeffrey Weissman. There are other scenes where you can tell it isn't him, but this is the most prominent one. Glover wasn't in the 2nd movie due to him and the producers not agreeing to a movie contract. All the scenes you see him clearly were taken from the first film. (i.e. where Marty is looking at him through the binoculars at the dance). (01:27:59)
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. (00:00:46)
Revealing mistake: When old Biff and young Biff are discussing the Sports Almanac in young Biff's car in the garage, young Biff tosses the Almanac into the air, landing in the back seat next to Marty. When it's thrown in the air his hand and the Almanac become transparent.
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.
Revealing mistake: When Griff and his gang are flying through the air and about to crash into the courthouse after the hover board chase, look at the girl on the left. Her legs hit one of the building's pillars, and her body rotates sideways, but she continues moving forward - obviously suspended by wires.
Revealing mistake: When Doc and Marty are driving on the Sky Way Doc says "damn this traffic," but in the reflection of the DeLorean's front window there are no cars. (00:29:14)
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. (01:23:55)
Revealing mistake: After Old Biff steals the DeLorean, he backs it up into a dumpster and several trash bags. The dumpster and the bags move before the DeLorean touches them. (00:32:50)
Revealing mistake: During the hoverboard chase in 2015 between Griff and his gang and Marty, you can plainly see the wires holding the gang up on their hoverboards in the wide shots as well as on the Jeep Wrangler before it lands in front of Marty.
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. (00:21:39)
Revealing mistake: When Marty is burning the almanac, during the lightening strike, the shadow of the post holding the hover-board off the ground is visible. - watch the hoverboard when the scene is illuminated by lightning strikes. The post holding up the hoverboard is also clearly visible when they pan around Marty as he is holding the burned rope from the Delorean.
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. (00:58:43)
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 in his left hand. (00:14:09)
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.
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