Continuity mistake: When Doc and Marty first see each other in 1885, just before they hug, Doc puts his hand on Marty's shoulder, in the next shot it's on his arm. (00:35:15)
Continuity mistake: When the DeLorean is being pulled by horses, the clouds in the sky change in the next shot. (00:38:50)
Factual error: Sierra #3, the locomotive used in BTTF 3, is a 1891 Rogers, despite this movie being set in 1885. (00:40:45)
Continuity mistake: Just after Doc saves Clara from the ravine, the carriage flips on its side and rolls. In the next shot, it's stopped rolling and is falling neatly straight down. (00:41:30)
Continuity mistake: When Marty is at the shooting gallery at the party and has his go, he points the gun in the wrong direction but still the targets go down. (00:54:18)
Continuity mistake: While at the dance Marty is showing off at the shooting gallery. After shooting all the targets he twirls the gun and hands it back to the booth operator with the barrel in his left hand. Very the next frame shows the gun in his right hand. (00:54:28)
Audio problem: When the very first song ends during the Festival scene, keep an eye on the guitarist to the right. His hand is still moving but no sound is coming out of his guitar. (00:56:55)
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)
Continuity mistake: When Doc realises that his name has disappeared from the tombstone, the date on the tombstone remains, but in the next shot, it disappears. (01:07:47)
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: At the bar Doc says that in the future people run for fun. When the guys playing poker laugh at him, the bartender magically appears next to them between shots. (01:17:43)
Continuity mistake: After the bartender prepares the wake-up juice, the two men by the table behind, plus the smoke coming out of the ashtray, disappear. When Marty pours the liquid the men reappear. (01:19:43)
Revealing mistake: When Marty pours the wake-up juice through the funnel into Doc's mouth, in the close-up the funnel is straight, but we can't see Doc. From a different angle where we do see the funnel stuck in his mouth, check the right side of the screen: The funnel is actually skewed (and seemingly it must have a tap) and the liquid is falling out in large amounts to prevent the actor from drinking it. (01:20:04)
Continuity mistake: When Doc drinks the 'wake up juice', he runs out and puts his head in the horse trough full of water. However, after a shot to Mad Dog Tannen, his shirt is mysteriously dry again. (01:20:05)
Continuity mistake: When Doc has just consumed the 'wake up juice' and runs out of the bar and dunks his head in the horse trough, Marty and the bartender come out after him. You can see that Doc is now soaked and a trail of water is on the ground behind him from a previous take of Marty dragging him back in the bar, which hasn't happened yet at this stage. Only really noticeable in the cropped version. (01:20:05)
Other mistake: When Clara is waiting for the train to stop at the train station and the train has steam coming from in front of the wheels, as it passes by the camera it jolts the camera. (01:20:20)
Continuity mistake: After Clara pulls the emergency pull cord she jumps out of the train with her gloves on. From the immediate opposite angle her hands are bare. (01:26:04)
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.
Continuity mistake: When Marty is shot by Buford, he is thrown off his feet and lands on his back. As he does, his poncho flips up, showing his shirt all the up to the middle of his chest. The stove door he is supposedly using as a bulletproof chestplate is conspicuously missing. (01:27:20)
