Continuity mistake: When the Doc of 1955 is about to send Marty back to 1885, he turns the time circuits on. However, if you look at the time circuits after Marty has reversed, you can see that they are turned off. (00:17:15)
Visible crew/equipment: When Doc's about to send Marty back to 1885 from 1955 at the open air cinema, you can see the shadow of the camera sticking out from the shadow of the building - it moves as the camera tracks Doc over to the car.
Revealing mistake: When Marty is in the cave with the bear, there are two non-consecutive shots from behind the bear. You can see scruffy, fake-looking fur on the bear in those two shots - obviously someone in a black fur costume. The rest of the shots use a real bear.
Continuity mistake: When Marty is walking into the 1885 Hill Valley, he walks past an A. Jones manure hauler, which is embedded in shadow. In the very next shot, not only is he several more metres down from the hauler, the hauler is in sunlight. (00:27:31)
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)
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: When Doc and Marty are chasing the train on horseback, Doc gets on the train first and his horse pulls forward. Then when the camera shows Marty on his horse, another horse can be seen in front of him. When Marty jumps on the train, his horse falls back. But in the next shot you can still see the horse as it moves off camera. (01:30:55)
Factual error: In the scene where Doc and Clara are playing with a telescope at night, the moon is seen twice. The phase of the moon appears to be a waning gibbous. Though the phase of the moon on the night of September 5th, 1885 at pacific time was waning, it was actually a waning crescent, a much different looking phase of the moon. I looked this up on the United States Naval Observatory website.
Factual error: The steam locomotive in the film is of a 4-6-0 wheel arrangement and is lettered as Central Pacific no. 131. The Central Pacific did have 4-6-0 engines in 1885, but according to the company's locomotive roster, the number 131 belonged to an engine of a 4-4-0 wheel arrangement. Also when the locomotive is uncoupled it shows a Janney (fist style coupler) - that style wasn't invented until 1890. The locomotive would have used a pin and link style connection.
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.
Continuity mistake: When Marty is about to go to 1885, before Doc shoots his gun, you can see the coils on the face of the DeLorean is brown, later when he is driving toward the screen, the coils are clear silver again.
Continuity mistake: When Doc and Marty get on the train at the end, Marty has some trouble getting on, and note that his hat falls off in the sequence, but in the next shot when Doc and Marty fall the hat is back on. (01:31:15)
Continuity mistake: When the Doc enters the destination time into the time circuits when he is sending Marty to 1885 from 1955, the time/date etc. Go in as he's typing it. However, in the previous films and later in the 3rd film when traveling back to 1985, the enter button must be pressed before anything appears for the destination time. (00:17:25)
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: At the end of the movie, when Doc and his family leave in their newly built train, the neon lights that flash over the front of the train go from left to right. In the next shot, where the train actually rises from the ground, they go in the opposite direction. (01:51:05)
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: In 1885 Hill Valley, the hills are shown to be almost directly behind the courthouse, however in the previous films there was some distance between the two. This was likely due to the different shooting locations. (00:26:40)
Chosen answer: Yes. A "googol" is the number 10 raised to the 100th power, or a 1 followed by 100 zeroes. A "googolplex" is an even larger number - 10 raised to the power of a "googol", or represented as 1 followed by a "googol" zeroes.
BGraz