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)
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)
Other mistake: A book that thin would not be able to hold even one year of statistics of all sports at the detail level purported in the film. When we see the pages (for example in the tunnel, when the book is in the wind screen of the car) we can see that the font is quite large, and there are even text sections - it is not all tables and numbers. And it can't just be regional results, since we hear about a college football match, which is listed in the book.
Other mistake: Griff enters Cafe 80s and tells Grandpa to wax the car. Grandpa stands up and walks to the door, but Griff keeps looking at where he was standing before. This was due to the VistaGlide system of filming an empty space without stunts that could serve as cues as to where to look at.
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.