Factual error: Though extremely modest on today's standards, the dress worn by Clara to the hoedown shows far too much cleavage for the time. No schoolteacher would ever wear a dress like that in the 1880s.
Factual error: When Marty goes to meet Mad Dog Tannen for the duel, in the background on the right side is the current flag of California. But that flag did not become the state flag of California until 1911, so would not have been around in 1885.
Factual error: Standard gauge for railroad tracks in 1885 was 4ft 8 1/2 inches. The distance between the center of the wheels on a Delorean was 5ft 3 inches. The car would not fit on to the tracks, and we can see the wheels aren't recessed or otherwise altered.
Plot hole: Although Marty rips the fuel line, not all the fuel would have drained out of the tank. Only the fuel that was already in the fuel line would have leaked out, meaning it could simply be patched, bled and the engine would run without a problem.
Suggested correction: A cut fuel line could absolutely drain the tank. Even if not a direct route with respect to gravity, it could still create a siphon effect and drain the tank.
Other mistake: When the movie ends and Doc says, "It means your future hasn't been written yet," watch his son Verne behind. He looks towards the camera and seems unhappy, makes a beckoning gesture (presumably at the crew), and points at his crotch. Seemingly, the kid was having a hard time trying not to pee on himself. (01:50:39)
Visible crew/equipment: At the drive in theater, Marty says he'll put the boots on when he gets there. The camera pans to the left to follow Marty and Doc as they continue to talk. On the very far left side of the screen, the shadow of the boom mic briefly dips into frame. This happens just before Doc walks over to the DeLorean from his car. (00:16:35)
Continuity mistake: When McFly gets to the village in 1885, he walks beside the manure guy and the sun changes between shots.
Revealing mistake: At the festival, after Marty is given the gun he encounters Seamus and Maggie. Due to the Vistaglide camera effect where Marty was later added to the scene, she is unable to look at where Marty is supposed to be. This is very noticeable when Seamus says "buffoon" and Marty turns around and starts to walk, but Maggie keeps staring at where he was. Also when the scene ends and she looks at Marty and warns him to watch for the future, she's totally missing the spot.
Continuity mistake: Just when Marty parks inside the cave to hide from the Native Americans, one wearing a war bonnet jumps over the mount. When the angle changes, the man is gone and has been replaced by two riders with no war bonnets.
Visible crew/equipment: During the party scene, towards the end when Marty and Doc are discussing the duel and the fact the train may be late, you can briefly see a PA system center left of frame (bass cab with PA head on top and above it 2 tweeters side by side) it would make sense that a PA system be on set during the scenes so the band and dancers had a soundtrack to mime/dance to. (01:01:29)
Factual error: When Marty travels back to 1885 and meets the Native Americans, we see a view from inside the car with the digital speedometer. The car is still going over 70 when the Indians are less than 2 metres away from the front of the car. There is no way the car would be able to stop that quickly from that speed.
Continuity mistake: When Marty is about to race against Needles, he holds the wheel with both hands. An instant cut later, his hands are gone.
Revealing mistake: When Clara's dress rips off and is about to fall off the train, Doc saves her. One can notice it's not Christopher Lloyd but his stunt double. (01:41:17)
Continuity mistake: When Clara is about to leave Doc's workshop, the position of Doc's hand on the telescope alters each time we move from rear to front shot. (01:08:09)
Factual error: Preparing to run the model train in Doc's workshop, Marty connects the clamp to the positive terminal. But he actually connects to the white ceramic insulator. Doc correctly connects to the negative metal terminal. (01:10:01)
Continuity mistake: While Marty is outside the mine, the sun changes on the floor.
Continuity mistake: While Doc saves Marty from Biff the sun changes between shots.
Deliberate mistake: In the duel with Marty, when Tannen gets fancy spinning his gun, when he places it in the holster the grip is facing forward. They then use effects to turn the gun so the grip is facing back. (01:26:44)
Suggested correction: No, the gun is spinning on his finger and goes into the holster with the grip facing towards the back, which is completely consistent with the spin of the gun.
Revealing mistake: When Doc jumps from the horse to the train, right after he hurries Marty, the man on the horse is an obvious stunt double for Michael J. Fox.
