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.
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: 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.
Continuity mistake: When the DeLorean lands in Hilldale in 2015, just before it touches the ground look at the wheel nuts. They are in a plus sign position (around the black centre. However when the car lands the nuts have changed to a X shape position. The wheel nuts change to the X shape position from the plus shape in the next shot as the car lands.
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: Marty's hairstyle is different in the starting scene in BTTF2 to what it was in the scene at the end of BTTF (It is flatter).
Continuity mistake: When Marty gets locked in Biff's garage, we see that he can't push through the two garage doors because there's a lock on the other side. Yet, when Biff comes out to go to the Prom dance, he opens them in literally no time at all with no resistance. We don't even hear him unlock them.
Continuity mistake: When Old Biff is seen watching the hover board chase all the monitors on the wall in the Cafe 80's are turned off even though they were all on two shots prior.
Continuity mistake: When Griff pulls "Gramps" Biff out of the cafe 80's, as he swings back into the cafe notice that he swings around twice. (00:15:00)
Continuity mistake: When Biff is trying to run down Marty on the hoverboard, as he comes out of the tunnel there is a shot of the manure truck driving down the road. Cut camera angle to Biff missing Marty and hitting the truck, which is now at a dead stop, however there was not adequate time to do this and appears that the truck was driving then instantly stopped.
Continuity mistake: The small black pickup Marty is given at the start of this film is significantly different from the one he's given at the end of the first movie. Yet the moment is supposed to be the same one.
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: In the tunnel chase, watch the next shot after Biff laughs which shows Marty furiously using all his might to escape Biff. There is suddenly black on the walls around where Marty is.
Continuity mistake: When Marvin, the band member, goes to call Chuck Berry about Marty's "new sound", the telephone he uses has a black neoprene coiled handset cord. In the first BTTF, the phone had a straight, uncoiled brown cloth handset cord (seen when he says, "Well then listen to this." and holds the handset out toward the stage). This isn't because the cord was stretched out, as neoprene cords can't stretch so much that they look uncoiled.
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: When Doc drives Marty up behind Biff, just before they enter the tunnel and Marty grabs Biff's car, you see Doc acknowledge Marty and then turn off left, but as the camera pans back you see the car veer to the right. (01:30:45)
Continuity mistake: When Marty is at the dance in the first film, the room that the band leader rings his cousin from is on the right, but in this film the room is on the left. (01:26:09)
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.