Continuity mistake: At the beginning, when Marty is examining his truck in the garage, the left side of his shirt is tucked-in in the front, but in the first film, Marty's left shirt tail was hanging out. (00:00:36)
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: 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)
Continuity mistake: At the start, when Marty and Jennifer kiss, Marty turns toward his parents standing at the door. The screen door has three lower sections, and the house numbers are black. When Doc says, "Where we're going we don't need roads," it then cuts to Biff rushing out the door. Now, in this shot the changes include a screen door with four lower sections, stone wall differs, the house numbers are bright white, and a light fixture directly over the house numbers. (00:01:05 - 00:02:20)
Continuity mistake: At the start, the DeLorean hits the left dustbin and comes to a halt. At the other side of the street, there is only one car parking near to a house. From another angle there's a different red car parking on the other side of the street, and the tree that stood between both houses has gone. (00:01:15)
Continuity mistake: Right at the start of the movie where Doc Brown pulls up on the drive of Marty's house, we see a panning shot as Marty and Jennifer walk to the DeLorean - there are no vehicles parked on that side of the street. A moment later as Marty waves his hand in front of the Doc's glasses, there's a green VW bus parked almost outside the house. Then as the Doc's looking through the rubbish bin, the VW's moved further down the street. Then as the DeLorean backs off the drive, there's a yellow VW bus there instead. (00:01:20 - 00:02:05)
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: 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: Biff has exactly the same view towards the street as Marty and Jennifer, but the houses and cars on the other side of the street are totally different. (00:02:26)
Other mistake: When Doc, Marty and Jennifer have arrived in 2015 Doc tells them that the time of the day is 4:29pm. However the destination time is still shown on the time circuits' display from their time travel to the future - the two times should match, but the destination time has the bottom AM/PM light illuminated, while the current time has the top one lit. They didn't arrive or leave at 4:29am, so something's wrong somewhere. (00:04:42)
Continuity mistake: In the beginning when Doc takes Marty and Jennifer to 2015 for the first time, they are descending towards Hill Valley. Doc turns the steering wheel to the left but the car goes right and down. (00:05:53)
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)
Revealing mistake: When Doc and Marty arrive in 2015 when the car is landing in the alley you can see the mechanism that is lowering the car. Just as the scene switches from the front of the car (right after the headlights go out) to the back of the car, look between the car and the building at the same level as the back bumper. There is something that is moving down with the car. Also, the vents in the back of the DeLorean are different. (00:06:12)
Continuity mistake: When the rain stops after Marty and Doc first get out of the DeLorean in 2015, Doc looks up at the rapidly clearing sky. Directly above their heads the sun is shining brightly. However, if you look at the tops of the buildings you can see sunlight hitting them from the sides. I suppose we will have another sun by 2015? (00:06:42)
Continuity mistake: We see Marty putting on his right Nike boot but not his left. The camera cuts from his feet to his face saying "power laces, all right" then back to his feet, now both boots are on. (00:07:45)
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 Marty reads the paper with the news about his son's arrest, the way he holds the paper differs between shots. (00:09:44)
Continuity mistake: When Marty is waiting for Griff he orders a Pepsi and has trouble opening it, but at another angle it's out of his hands and on the table, then in the next shot he's trying to open it again. (00:13:41)
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)
Revealing mistake: When old Biff turns to face Griff, his elbow passes through Marty's Pepsi bottle. (00:14:41)
Answer: He is attempting to play the song "The Power Of Love" by Huey Lewis and The News. He was first seen playing it very well in the first Back To The Future movie when he was trying out for the school dance. Jennifer recognizes the song and is shocked that Marty who was so good at guitar was suddenly able to hardly play it.