Plot hole: In the beginning, Rachel and Loomis are referring to Rachel's mother as being Jamie's step mother and Rachel refers Jamie as being her step sister. Rachel's mother is Jamie's foster mother and Jamie is Rachel's foster sister as said in Halloween 4.
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: During Riggs' last fight with Adolph, Adolph holds the knife in his own hand as Riggs forces him to stab himself very slowly. At any point, Adolph could simply have let the knife fall or just tilt down before that happened. Nothing Riggs is doing is making such impossible. "Locking" his wrist does not prevent him from letting go of the knife. It only prevents him flexing his wrist. And this guy is a trained killer, fighting for his life. Better to battle for control of the knife than just let it kill you.
Plot hole: Right after the Joker goes into the cathedral with Vicky, Batman follows him with no one else entering in between the two parties. How then did Joker's goons get to the top of the cathedral before Batman or the chopper? They couldn't have planned it since Joker had to tell them where to pick him up, and they didn't know he would chase after Vicky to that particular location.
Plot hole: Bond goes after Lupe after she leaves the casino table, and they have quite a lengthy, hush-hush conversation. It's a casino, and there are cameras everywhere. Certainly, this would have been brought to Sanchez's attention since he was already leery of Bond and possibly Lupe.
Plot hole: David Sutton claims he's never read an autopsy report before yet earlier he said he moved to his new town to get away from all the murders he investigated, a job that would have required him to read autopsy reports.
Plot hole: We discover that Teresa the maid has been turned into some kind of living zombie at the end of the movie. She confronts Alex and Megan near the end, and they run away from her and then fight with the Puppet Master. The issue of Teresa is then never addressed in the rest of the film, she just simply disappears from the plot and the scene. (01:13:35)
Plot hole: Granted, Coffee is suffering from "high-pressure nervous syndrome," but if you're a Navy SEAL, wouldn't you notice whether or not your pistol has a magazine in it? Monk pulls the clip out of his jacket and starts knocking rounds out of it for the rig crew's benefit; the same dramatic effect could have been achieved with a handful of cartridges he could let slip through his fingers. You could argue that Monk didn't have enough time to unload the gun and replace the magazine, but in that case, I think the scene where Monk briefly had the gun should have been a bit longer. (Even had the empty magazine been IN the gun, Coffee still might notice the difference in weight, but that would be easier to suspend your disbelief for.) (01:43:53)
Plot hole: Megan is suspended because the police were not able to find the gun the robber had (stolen from Eugene at the scene) and believed that she had killed an unarmed man. This is very unrealistic because surely they would have interviewed all of the hostages/witnesses at the robbery and each would have easily confirmed that the robber had a gun, and possibly that Eugene might have been missing right after the incident.
Plot hole: The odd couple tails the suspicious guy bossing around the car company mogul and run a check on the license plate. The response is "Car registered H.W. Scully. No known residence, only address PO Box 25047 Detroit." They tail him a bit, end up in the bowling alley and lose him in the commotion. After that, Leno asks his buddy at the district if he has "anything on that guy Scully", and the answer is "This guy has done anything but rape Bambi. Several weapons charges, assault and battery, he beat a murder rap in Sarasota" etc. And of course, an address. (00:56:35 - 01:01:35)
Plot hole: After Ann retrieves Tibor Zoldan's music box from the pawn shop, she finds the incriminating photos of her Nazi father, Michael Laszlo, hidden in it. Even though Zoldan (who had died) could have redeemed the music box from the pawn shop within a certain time period, it makes no sense that he would have pawned it with the photos left inside. The photos were how Zoldan was able to blackmail Laszlo. It's highly unlikely he would have forgotten they were in there.
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.