Corrected entry: There are two Time Machines in the Old West (1885). One is with Marty and the Doc. The other is in the mine where Doc put it. It has to be there because if it's not, Marty and the 1955 Doc could've never got it, repaired it, and sent Marty to 1885 to save the Doc of 1985. 1985 Doc and Marty could've saved themselves a LOT of trouble by going to the Time Machine in the mine (in 1885) and transporting the gas from it into the other Time Machine (and, of course, repair the fuel line).
Corrected entry: This would only be noticeable to the true movie geek, but when Doc and Marty are looking at the map to plan their trip back to 1985, Clara is standing in the background waiting for Doc to pick her up as he promised he would do. Since he doesn't show, she apparently rents a wagon herself and. . . well, the rest is history.
Corrected entry: When Marty leaves for the future at the beginning of Part II, Biff is in his red tracksuit about to wash their cars and Marty's black truck is in the garage. Then in Parts II and III, Marty and Doc make plenty of changes in the past and Doc staying in 1885 would greatly change things on Marty's return to 1985. But when Marty's returns to 1985 at the end of Part III, Biff is still in his red tracksuit about to wash the cars and the same black truck is still in the garage.
Corrected entry: If the 1955 Doc saw his tomb and sent Marty to 1885 he shouldn't be surprised to see him in 1885, since the Doc that's in 1885 is the same one who sent him there in 55, and hence has already seen his tomb. Some people argue it's not the same Doc, but time travelling doesn't magically create a new human being! The 1955 Doc sends Marty back to 1885, ages 30 years, invents a time machine, travels around, and ends up in 1885. 30 years may dull the memory a bit, but I think you'd remember sending someone into the past to rescue yourself...
Corrected entry: In the last scene in 1885 where Marty, Doc, and Clara are on the train racing toward the bridge, Marty keeps yelling at Doc on his walkie talkie to tell him the gauge readings. But after Doc sees Clara on the train, he tries to rescue her and Doc is no longer holding his walkie-talkie. Doc is holding on to the train with one hand while reaching for Clara with the other. Doc's walkie-talkie is no longer anywhere in sight. Marty sees all this but he nevertheless keeps yelling into his walkie-talkie right to the end as if that is how Doc could hear him. Marty does this even when he is facing forward all by himself in the DeLorean. Who is Marty talking to on the radio? It can't be Doc.
Corrected entry: Doc's tombstone was placed by his "beloved Clara". However, that was discovered (and carved) before Marty went back and changed the past by him and Doc rescuing Clara. So how did Doc meet Clara without Marty being there, given that the ravine was already called Clayton ravine, meaning that by definition she must have fallen in? Bit of a paradox, methinks. Even assuming he met her at the station, then after that her carriage fell into the ravine, the timings just don't work at all - she couldn't have fallen for him in minutes, and/or couldn't have had her name on the gravestone given that she fell in the ravine before Doc was shot.
Corrected entry: When the 1955 Doc sends Marty back to 1885 he points to a cave where to hide the DeLorian left of the track that Marty takes (directly towards that cinema screen). In 1885 then, Marty takes a U-Turn to flee from the Native Americans. Assuming a 180 degree turn, the cave to hide the car must be on the right hand side. However, the cave appears directly in front of him.
Corrected entry: There really should have been no reason for Marty to risk going back to 1885 to rescue Doc. Obviously, 1985 Doc being killed by Tannen, did not negate his eventual existence (as evidenced by 1955 Doc still existing). It's understandable Marty would want to prevent his friend being murdered, but when they learned of this fact in 1955, this event had already happened 70 years ago. 1955 Doc surely would have realized the pointlessness of Marty going to rescue his future self in the past when 1955 Doc would eventually become 1985 Doc again through the natural course of time. In light of this, surely 1955 Doc would have explained to Marty there was no reason to go back and possible cause more damage to the Space-Time Continuum.
Corrected entry: Doc Brown can build a time machine but is unable to figure out how to build a still to extract pure ethanol from Whisky in order to use as fuel for the DeLorean. He stated to Clara that he is a "student of all science" so this should include chemistry. Chemists know how to build a still. Doc, however, tried to fuel the car with strong Whisky without distilling it.
Corrected entry: Freeze frame is recommended when trying to catch this one. When Clara rides out of the stable her hat falls off, however in the next shot of her turning the corner you can see the silhouette of the brim, indicating that it has returned to her head.
Corrected entry: Right toward the end of the movie, a split second before the Train crashes into the Delorean, underneath the Car you can see that a fireball bursts, causing the effect.
Corrected entry: When the DeLorean is destroyed by the train at the end, why does the train smash through it, instead of just pushing it back down the tracks? Even if Marty had put on a parking brake (which we don't see him doing) it doesn't seem like it would be strong enough to withstand the force of the train. Even cars that are sitting perpendicularly on railroad tracks get pushed along by fast-moving trains, so why not a car that's specially designed to move on the tracks?
Corrected entry: After the DeLorean is destroyed by the train, Marty goes to get his truck, picks up Jennifer, almost races with Needles, and then finally returns to the scene of the accident. Isn't it odd that a car was hit by a train, yet there are no emergency vehicles anywhere around, or that the train didn't stop afterward?
Corrected entry: When the locomotive crashes from the bridge, there is a huge explosion (may have been added digitally in a later version). However, there are no explosives in a steam-engine, only water and wood.