Revealing mistake: When Doc flies the DeLorean up and the wheels get stuck to a banner string, the close-ups of Doc show a lit cockpit, but the shots from the outside show it's dark and one can't really see who's at the wheel. However, for a couple of seconds you get to see it's not Doc but a stunt double or a stiff dummy. (01:23:25)
Continuity mistake: Notice Marty's hair in the scene when he's playing "Johnny B. Goode." It's quite larger than when he says, some seconds later, "I guess that you're not ready for it yet." That's because when he says that, it's a recycled shot from the original BTTF, whereas most shots showing he's playing the guitar are newly made for BTTF2. However, Marty's hair is considerably longer in the new shots. (01:27:19)
Revealing mistake: In the scene where Doc Brown is following Biff into the tunnel, as Marty starts to pass between the cars on his hoverboard, one of the wheels on the rig supporting the hovering DeLorean is visible. (01:30:30)
Deliberate mistake: When Old Biff and Young Biff are sitting in Biff's car, the car's rear-view mirror vanishes during all shots of Old and Young Biffs together. Presumably this made the split-screen effect easier to do.
Visible crew/equipment: Right as Young Biff starts to chase after Marty through the tunnel, there is a shot of the wheel spinning around, and you can see the camera crew reflected in the hubcap. (01:29:45)
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: In the Café 80's just after you hear Marty say, "It is Wild Gunman", notice the three people sitting in the second booth from the back wall. When the scene changes to Marty throwing his hat on top of the game, you can see that the people have changed tables. You can also see that they are gone again when it shows a close-up of the two kids. (00:15:18)
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 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)
Visible crew/equipment: At the end of the famous court yard chase scene when Griff and gang crash through the courthouse windows, you can see in plain sight the reflection of cables and the mechanisms that held them in the air. [Corrected in 25th Anniversary edition.] (00:20:55)
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: 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)
Visible crew/equipment: If you look at the money case that Doc brings with him that has all the different types of money for each year, you will see where it says 1864 the reflection on the glass or plastic of what appears to be a tripod most likely for lighting. (01:03:47)
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)
Revealing mistake: As Doc and Marty go to the soon-to-be Lyon Estates in 1955, the DeLorean hovers over the camera. Look at the roof. You can see 3-4 wires attached to the time machine.
Continuity mistake: Before Griff shows up in the cafe, the Pepsi bottle's label is facing the counter. A second later, when Griff appears, the label is facing the door. (00:14:47)
Revealing mistake: When Marty goes to burn the almanac he flips the hover-board to his hand. The board begins to flip up before Marty touches it and even so, he doesn't touch it at all, he steps his foot behind it. (01:35:19)
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)
Revealing mistake: When old Biff and young Biff are discussing the Sports Almanac in young Biff's car in the garage, young Biff tosses the Almanac into the air, landing in the back seat next to Marty. When it's thrown in the air his hand and the Almanac become transparent.
Continuity mistake: At the end of the preview for BTTF3, Marty and the Doc are standing in a position for their photo to be taken. The photo is taken, but on the photo their hands are in different positions to what they were when the photo was being taken.
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.