Continuity mistake: When Marty arrives in 1955, he lands on Peabody's farm and rams through the pine tree he then hits a scarecrow which flies over the top of his car. Then as they show it from the inside the scarecrow slams into the windshield - but how can it slam into his windshield if it is already over the car? (00:31:30)
Continuity mistake: When Marty's been thrown back by the amp, the papers on top of him change between the first and second shot. Precisely a Polaroid photo, a poster/map and a vase are missing. (00:04:20)
Continuity mistake: After George lays Biff out, Marty pulls the family photo from his pocket. The way he holds it suddenly changes, judging by the position of his index finger as the shots change. (01:23:45)
Continuity mistake: At Twin Pines Mall, when Marty is recording Doc Brown, Marty is raising the video camera to record Doc Brown saying, "No, no, no, this sucker's electrical," but when this piece of film is played back in the Doc's garage in 1955 it appears as if Marty was already recording when the Doc starts speaking. (00:25:15 - 00:50:40)
Continuity mistake: At the beginning of the film, when sending the DeLorean into the future, the wide shot shows the DeLorean heading for Marty and Doc then disappears leaving a trail of fire. Notice the cut as the car park goes from being dry to wet with puddles. (00:20:00)
Continuity mistake: When Marty is at the "Start" line waiting for the alarm clock to ring, he moves up his return time and the engine stalls out. Next we see Marty turning an ignition key with several other keys on a chain like the ones used on ceiling fans with the little balls and a plastic fob with the paper insert that reads 555-1138 Out-A-Time on it. After Marty arrives back In 1985 in town square, he pauses and gets out for a moment. While returning the the car he puts the DeLorean in drive and again the car stalls out. Again we see Marty restarting the DeLorean, we see a different set of keys. The key mysteriously changed in the time travel to an after market type with a thin wire ring on it, no key fob and the other keys are missing. (01:38:43 - 01:42:44)
Continuity mistake: When the Doc is trying to pull the cable to connect it the cable is stuck and when he pulls the cable hard the other connection breaks off. Watch when it breaks off you cn see that after Doc pulled it the cable gave him more length and would surely get Doc's arm to the other cable but when they show the Doc a second after he hasn't moved one bit. (01:39:35)
Continuity mistake: When Marty is playing at the dance the picture on the guitar disappears in some shots.
Continuity mistake: After Doc accidentally unplugs the cable and is sending Marty back, he unplugs the cable 15 seconds later and it shows Doc, and the clock's hand change. A split second later the clock's hand changes again.
Continuity mistake: While being chased by Biff, after Marty knocks down a man and a woman, the man stands up and helps the woman stand up, but a frame later he is rolling on the floor with the woman on top of him.
Continuity mistake: After the shelves fall on top of Marty, the rubbish on top of him changes between shots. (00:04:10)
Continuity mistake: George McFly's holding a pen in his right hand, then a cut to a wide shot reveals him not holding a pen as he rests his arm away from his body on the table. Then when the camera cuts back his arm is back in front of him holding the pen. (00:14:10)
Continuity mistake: Marty is hitching a ride on the back of a jeep, a wide shot shows him going past and beyond the 'Lou's Aerobic Fitness Center' next to the Texaco garage, there's a cut to his skateboard then back to a medium shot of him going pass the same fitness centre again, this time waving to the people inside. (00:06:00)
Continuity mistake: When Doc starts up the DeLorean watch the remote when he says to Marty "not me, the car, the car." The counter is static at 00 but the car is moving. (00:19:40)
Continuity mistake: Cafe 1955, when Marty enters for first time. The arrangement of the coats on the rack to the right hand side of the cafe doors change upon Marty's initial entrance and returning to the counter after using the phone booth. (00:35:50 - 00:36:40)
Continuity mistake: In the cafe, the menu in front of Marty changes after Biff leaves. It starts looking like an ice-cream menu, with swirly lines at the top, but just before the future mayor comes into shot, the design on the front changes completely. (00:38:15)
Continuity mistake: In the scene where Marty tells George his plan for getting Lorraine to go to the dance with him, Marty's shirt is sometimes tucked in and sometimes not. (01:12:40)
Continuity mistake: Beginning of film. When Marty arrives at Twin Pines as the Doc requested he cruises up alongside Einstein on his skateboard, saying "Where's the doc, boy?" There's a cut to the truck opening, then back to Marty, and for some strange reason the yellow wheels on his skateboard have turned pink, but the reflection on the wet car park is still yellow. It cuts back to the DeLorean coming out of the truck then back to Marty, and his wheels are yellow again. This doesn't happen in the VHS version. (00:17:40)
Continuity mistake: As the Peabodys try to determine what crashed into their barn, the DeLorean's left indicator flashes throughout. When the family is onscreen, the barn's interior goes completely dark between flashes - whereas there should be illumination from the headlights as well. Compared to a later point in time where Marty opens the barn door to get shot at, it's not hard to notice how bright the headlights are. (00:31:50)
Continuity mistake: When the wind unplugs the cable connected to the tower and Doc runs towards it, the position of the cable changes between one shot and the other in barely half a second.
Chosen answer: I think that is *exactly* what he is thinking. He realizes that by standing up to Biff, George may have irrevocably changed his personal future, and therefore affected Marty's future as well. This is exactly the sort of thing Doc was so eager to prevent by refusing to hear any information about the future.