Continuity mistake: While Marty is admiring the new truck, Jennifer walks onscreen. Once the angle changes, Marty's left arm is resting on an entirely different part of the door. (01:49:50)
Continuity mistake: When Biff and gang are chasing Marty on the skateboard, they eventually crash into the rear of a manure-hauling dump truck. When the car swerves left in an attempt to miss it, it will obviously pull somewhat to the left. In the shot just before impact, you can see that just the rear half of the car should impact the truck, but in the next shot the car rests perfectly centered behind the dumps open load. (01:07:25)
Continuity mistake: In the whole where When Biff is chasing Marty (on the 'skateboard') around the Town Square, the sun changes at every shot.
Continuity mistake: When Doc finds Marty's letter in his pocket, the big hook fastened to the DeLorean's passenger side is seen facing the sky. Pretty soon Marty slides across the hood and into the DeLorean, but now at this point it's facing the ground instead. (01:33:35 - 01:35:50)
Continuity mistake: When the McFlys are together in their kitchen, Lorraine asks everyone to drop Uncle Joey a line. At this point George lowers his left hand and seemingly reaches for silverware on the table. But between shots, he lifts his hand back to its original position all of a sudden. (00:15:00)
Continuity mistake: Before Lorraine brings Joey's cake, George leaves a bunch of crumbs on the table and starts to watch TV and do some numbers. After the cake lands on the table, the place is spotless.
Continuity mistake: When Doc syncs his stopwatch and the dog's, he holds them away from their bodies. A frame later his hands are close to his chest and there's dog hair all around.
Continuity mistake: After Joey's cake lands on the table, George puts his arm to the right of the cereal package. A frame later it's on the left side and holding a pencil.
Continuity mistake: After escaping from the Peabodys, Marty stops the car in the middle of the road and exits with his hair all messed up. A frame later it's brushed.
Continuity mistake: When Marty gets hit by the car and wakes up in Lorraine's bedroom. She constantly calls him Calvin. Marty says "Calvin, why do you keep calling me Calvin"? When he says this he puts his arm on his lap. In the next shot behind him when Lorraine replies, his hand/arm is by his side. (00:43:40)
Continuity mistake: When George is in the school cafeteria talking to Marty about science fiction stories, he holds the notepad differently with each changing shot. (00:58:55)
Continuity mistake: While Marty's 1950s family is having dinner, the milk bottle's level is one mark below the red printed label. A second later it's 5 centimeters above the red mark.
Continuity mistake: When Doc puts Einstein in the car, the dog turns his head inside the car. A frame later, from a new angle, it's outside the car.
Continuity mistake: When Marty was recording, Doc never hesitated between "reaction" and "to", then when they are watching the tape, he hesitates a little.
Continuity mistake: When Goldy (or Golby?) says, "Mayooorr", George is facing Marty. Then when Goldy starts saying, "Now that's a good idea," George is facing Goldy, without any motion of turning. (00:40:35)
Continuity mistake: When Doc and Marty are watching the DeLorean burn out from across the vacant Twin Pines Mall parking lot, the radio control unit's antenna is seen lowered one shot then fully raised in the next, despite Doc not having touched it. (00:21:20)
Continuity mistake: When Marty sees the Lyon Estate works there's a mild breeze blowing. Just a frame later, from a new angle, there's a very strong wind.
Continuity mistake: The time machine is gone for 1 minute and 21 seconds - not just 1 minute. (00:22:00 - 00:23:20)
Continuity mistake: George's hair fixes itself when he tells Marty about Darth Vader coming down to visit him. It's initially messier.
Continuity mistake: Before Marty kicks the speaker while playing Johnny B Goode, there is a case of Pepsi below it which disappears between shots.
Answer: Video tape system back then could output an NTSC video signal, just like broadcast at the time, and up to HD in the 2000s. Usually there was a switch on the video device to change the output frequency between channels 3 or 4. Depending on what was an open channel in your area.