Continuity mistake: If you look very closely, after Marty flips on the little girl's hover board and is laying on his stomach, when he goes to get up you can see that he's wearing regular lace style tennis shoes and not the auto-lace high tops that Doc gives him as soon as they get to 2015. You may need to pause it to see them. (00:18:56)
Continuity mistake: When Marty is being chased by Griff's hover board gang, Marty grabs rope from a car. Just after, he has to avoid an oncoming car. One second he's holding the rope, the very next shot as he goes over the bonnet, the rope's gone. (00:19:38)
Continuity mistake: When Marty avoids Griff and his thugs by jumping in the water off his hover board, his hair gets wet. After his coat dries his hair he rubs his hand through it. It gets all messy and crazy and uncombed. The very next shot shows him talking to the girls with perfectly combed hair. (00:21:33)
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)
Continuity mistake: When the police car arrives in Hilldale, there are no door handles on the car, but in the next shot, some appear. (00:27:16)
Continuity mistake: In the scene where the 1985 Jennifer is in her 2015 home and says "I get married in the Chapel-O-Love?" right before she picks up the picture note the Chapel-O-Love picture and frame on the bookcase behind her. After she picks it up and drops it leaving it on the floor she looks for a place to hide. Later on when the old Lorraine and George are in the living room, take another look at the picture on the bookcase; the Chapel picture is set back up. You could argue Lorraine would have had enough time to set it back up. But look at the Chapel picture; it's inserted in the frame upside down. You can tell by looking at the frame stand behind it. Meaning someone physically removed the picture and put it that way. (00:28:18)
Continuity mistake: When 2015-Lorraine takes the pizza out of the oven, the amount of green peppers decreases between shots. (00:33:22)
Continuity mistake: When Marty, Jennifer, and Doc head to the 2015 Hill Valley, part of the text on the dashboard is faded, but when they travel back to 1985, the text is completely intact. Additionally, in the second shot, the 'Last Time Departed' is mentioned as Nov. 12, 1955, despite it being Oct. 26, 1985. (00:38:22)
Continuity mistake: In Biff's apartment in alternate 1985, while Biff and Lorraine are arguing, he knocks her to the ground and both her shoes come off. After Biff punches Marty in the stomach, it then switches again to show Lorraine's reaction. As it does this, we see her on the floor but her shoes are back on her feet. She could have had enough time to put them back on possibly, but it seems unlikely that she would have done so as she is lying in the same position as when she was knocked down. (00:48:18)
Continuity mistake: When Marty confronts Biff in the alternate 1985 in the hot tub, the girls change positions. The blonde was on his left and the brunette was on his right. When they see Marty and scream, they move forward but there is no way they can switch that fast. (00:55:40)
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: After the roof scene in alternate 1985, the time circuits show the Nov. 12, 1955 readout before Doc enters it. (You can hear him still pressing the buttons afterwards). (00:59:00)
Continuity mistake: When Marty hides in Biff's car as he walks over to it after leaving the auto shop, the left visor on the car is raised, but in the next shot, it is lowered. (01:05:53)
Continuity mistake: When Biff is trying to get Lorraine to go to the dance with him, (the bit when Lorraine is looking at her dress) you see the same extra walking past her about three times. (01:06:07)
Continuity mistake: When Old Biff drives into the garage with Young Biff in the passenger seat, Young Biff (body double) is not wearing a belt, but the belt reappears when they exit the car and Old Biff places the Sports Almanac in Young Biff's back pocket. Also, the Levi's small red tab is now missing from that pocket. (01:07:55 - 01:10:05)
Continuity mistake: In the part where Marty is trapped in Biff's garage, Biff's rear view mirror on his car is at the very top of his windshield. Later on when Marty is holding on to the car with the hover board the rear view mirror is in the middle of his windshield. The rear-view mirror was lowered about six inches or so. (01:10:12)
Continuity mistake: When Marty is in Mr. Strickland's office trying to take back the almanac, it shows a wide view of the office with two flags right next to each other. After cutting to a close up view of Marty they are apart, then Marty moves them together. (01:19:20)
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: When Doc turns the time circuits on as he is about to head over to the high school, the 'Destination Time' is under 'Last Time Departed', but in the next shot, it is back to being over it like normal. It should also be noted that the month is represented by a number in the first shot, but with letters in the second. Additionally, one of the LED's is burnt out in the first, but is back to being lit in the second. (01:23:30)
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)
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.