Plot hole: If Old Biff changed his past and went back to 2015, he goes back to HIS future, not the bad future, but Doc later tells Marty that if he were to go to the future to stop Biff from taking the almanac, he'd go to the bad future, so Old Biff technically shouldn't have been able to return to "his" future at all.
Plot hole: After Biff steals the DeLorean and travels back to 1955 with the Sports Almanac the movie states that he creates an alternate timeline. Doc Brown does a good job of illustrating this on a chalkboard. So if, as Doc Brown states, Biff created an alternate timeline, then when he travels back to 2015 it would be the alternate 2015. Even if the original timeline stays intact with Doc and Marty in it, they'd be trapped there because Biff took the Delorean and returned it to the new timeline, not the one they're in.
Suggested correction: When Doc explains to Marty the events that changed the timeline from 1955 onwards, several hours have passed since they returned from the (original) 2015. Old Biff likely returned to the original 2015 not long after after giving young Biff the almanac, so it is entirely plausible that not enough time had passed for the alternate version of 2015 to take ahold. When Doc tells Marty if they were to go back to the future they would be going to an alternate future, enough time had passed for the future to turn into the alternate future.
Trivia: The voice of Biff's grandmother was provided by Thomas F. Wilson.
Suggested correction: According to The IMDB website, Charles Fleischer who played Terry the Tow Truck Driver and the old man taking donations in 2015, provided the voice of Gertrude Tannen.
And other sites list Wilson as the voice of Gertrude. Neither men are credited for the role and I have yet to see any site list a source for the information.
Https://www.imdb.com/name/nm0281486/otherworks.
IMDB is not considered a good source. Plus, on that site the information is given as trivia without a source, any crackpot could have claimed that.
Well until you can show me a confirmed source stating it was anyone else, my correction stands.
But IMDB isn't a confirmed source. Here's a site that lists Wilson as the voice actor, so by your logic the trivia entry stands. https://backtothefuture.fandom.com/wiki/Gertrude_Tannen.
On the audio commentary it is stated that the voice of Grandma Tannen is Fleischer. This is stated right after we hear "her" voice when the outside of her house is being shown.
Trivia: When Marty discovers that he is in an alternate Hill Valley, there was going to be a scene with him discovering that his sister Linda was a prostitute and his brother Dave a homeless drunk living on the streets. These scenes were scrapped as Wendie Jo Sperber, who played Linda, was pregnant at the time.
Suggested correction: Very unlikely for a family film, and needs to be supported by some evidence such as a published interview with a film principal. This is a ridiculous entry and should be removed.
There was, in fact, a deleted scene in which Marty comes across Dave, who has since become an alcoholic, in the town square. While it is true that Wendie Jo Sperber was pregnant and thus unavailable at the time the film was made, I've never heard of anything scripted about Linda having turned to prostitution.
If you have the DVD or the Blu-Ray, watch the movie with the Trivia Track on. It will confirm that Wendie was originally supposed to appear as her character Linda who had become a prostitute.
Plot hole: When Old Biff goes back to 1955 to give himself the almanac, he comes back in the Delorean to the version of 2015 that he left, not the other, skewed version in which he is rich. Everything in 1955 should have changed around the Doc and Marty, as the Doc tells Marty everything will change around Jennifer and Einstein later on in 1985, when Marty and the Doc go back to restore normalcy. George is alive in this future, so we know the skewed version hasn't taken hold.
Suggested correction: It is established in the movies that the effects of the timeline being changed are not immediate. For example, in the first movie, in the photo of Marty and his siblings, it takes some time for them to vanish from it. When Doc tells Marty everything will change around Jennifer and Einstein when the timeline is restored, I don't think he meant the future will be restored to normalcy right that second, but more after an ample amount of time has passed for everything to be right. After Old Biff gave the almanac to young Biff, there was ample time for him to return to the original 2015 before it changed into the alternate 2015.
That would mean when they restored the timelines it would have taken time for it to adjust again, but it didn't. The new timeline was created but the old one remained because Marty and Doc were still in the original future. However, even though they are not in their original timeline it doesn't make sense for them to still be there, the timeline should have been erased or else old Biff wouldn't be erased either. Again though, a copy of another plot hole, which one is the oldest and original?
Continuity mistake: At the beginning of the movie when Doc smashes through the trash cans, his glasses are not on. In the shot where he steps out of the car they are.
Suggested correction: He put them on in between shots.
Suggested correction: The effects of the past being altered may not have happened immediately. It is possible that it took time for the timelines to adjust to the changes of events, meaning enough time would have passed to change 1985 when they return, but not enough time could have passed to change 2015. By the time Doc says if they went back to 2015 they would be going to an alternate future, some time has passed, so the effects of the past being altered and taking ahold in 2015 and altering it are more likely to have occurred by then.
Casual Person
Here is what you say: "perhaps it took time for the time lines to adjust." What kind of time would timelines take? Time is time, it doesn't take time to change the timeline. That doesn't make any sense. Some people claim it was the DeLorean itself that came back to its own original timeline and only then reset itself in the new one, but then the new timelines being erased later on wouldn't have happened either. So its a genuine plot hole.
lionhead
It's established in the first film that it takes time for the changes to take effect. Marty and his siblings slowly disappear from the photo, rather than instantly. Although the scene in BTTF2 was deleted, it was filmed showing Biff dying and slowly fading away after his return to his present.
Yet they were restored instantly without any outside influence at the end of the movie. There are a lot of things wrong with this movie and the first one. Old Biff disappearing should mean that Marty and Doc should slowely disappear as well, even the DeLorean. But they didn't, that doesn't make any sense. The point is there is a plot hole, somewhere. To know where all you can do is look at it logically and then you automatically come up with Old Biff going back to the future but not the alternate future. If he did there wouldn't have been a movie, but that's the plot hole.
lionhead
The timeline didn't change until he made his first bet which was some years I think after receiving it. He immediately travelled forward after giving the act, meaning he will still jump forward to the original future.
The timelines would instantly change, and Old Biff couldn't possibly have returned to "normal" 2015. It's just a poorly-thought-out time travel plot hole (or a deliberate error to expedite the storyline).
Charles Austin Miller
Suggested correction: In context, Doc was saying that they couldn't return to 2015 to stop Biff from stealing the time machine, because Biff didn't steal the time machine in the alternate 2015, he only stole it in the original 2015. Marty and Doc didn't stay long enough in 2015 after Biff returned, and that's why they didn't see any differences. Also, though they were unaware of it, Biff was dead in the alternate 2015, so the disasters he caused might have reverted back after his death.