Continuity mistake: When Marty is on Strickland's porch with the shotgun pointed at him, his hands go from in the air to down by his waist in a split second.
Continuity mistake: Int the courtyard chase scene when Griff and gang crash through the courthouse windows, the sun's position changes in every shot.
Continuity mistake: When Strickland is hitting Biff with the Almanac in the parking lot, the way the book is folded changes between shots.
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: While Marty gets to 2015 it's raining and the streets are wet, but when Griff's gang is chasing him the streets are dry.
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)
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.