Continuity mistake: As paramedics inflate Jordan's assistant Stan Podolak following his lone bucket, the scoreboard shows the Monstars ahead 77-67 with 10 seconds remaining. Yet following his treatment and the surprise entrance of Bill Murray, the score has changed to 77-76 with no time having elapsed.
Continuity mistake: In the scene where the Nemesis kills the 13 S.T.A.R.S members, after it blows up the sniper on the roof an additional S.T.A.R.S member can be seen outside the building on the right who wasn't there earlier. He then disappears shortly afterwards.
Continuity mistake: Near the beginning, when Banner arrives at the lab with his helmet on, he removes the helmet when he sees Betty. A close-up of his face is shown and his hair is messy with part of his fringe on his forehead. Betty is shown quickly, then the same shot of Banner is shown again, but this time his hair is perfect.
Character mistake: When they're on the island and the INGen helicopters are flying in, Jeff Goldblum takes the binoculars and looks through the wrong end. (00:32:30)
Other mistake: So let me get this straight: Optimus Prime has Energon Swords hidden in both of his arms, with which he can slice through an entire Decepticon in one chop, yet when he is hanging from a few wires, despite having one arm entirely free, he can't cut himself loose?
Continuity mistake: Notice Marty's hair in the scene when he's playing "Johnny B. Goode." It's quite larger than when he says, some seconds later, "I guess that you're not ready for it yet." That's because when he says that, it's a recycled shot from the original BTTF, whereas most shots showing he's playing the guitar are newly made for BTTF2. However, Marty's hair is considerably longer in the new shots. (01:27:19)
Continuity mistake: During the scene in the mall when the girl is in the dressing room, she keeps putting on more layers of clothing. But as they are shooting different shots she will have a hot pink shirt on, then it is back on the hanger, and then back on again. It switches back and forth many times. (01:00:50)
Continuity mistake: When Sly is in the attic, and the fake power dude comes up to kidnap him, Sly uses a ski to "hit him in the gonads". But if you look closely at his face, the guy who was hit in the nuts wasn't the same guy that fell down the stairs.
Plot hole: They aren't able to travel before the birth of their children or their children become different. Despite that, he visits his father one last time before their third child is born. His Dad and he then travel back to when he was a kid, which would have changed his first 2 kids.
Suggested correction: This is not necessarily inconsistent with the movie's time travel logic. Since Tim goes back in time to visit his father and inside that particular time travel, they use time travel to go back to a day where Tim was a child. Then, they return back to the original time travel where they're playing ping pong. At that moment, both of Tim's kids were already born. So, when he returns from the ping pong scene, there's no change regarding his kids.
Also, paraphrasing the Dad, he said they aren't going to change anything. It is more like reliving a memory than changing the future to get what you want. The Dad couldn't go back to not smoke before his kids were born because that was major and would have changed the course of his life; walking a bit differently on a secluded beach that you walked on in your younger days (as long as you spent the same amount of time - presumably) would not alter the trajectory of one's life.
The slightest change in 1 second of being on the beach would absolutely affect his kids. A man produces 1500 new sperm every minute. Altering the timeline and delaying every event that is going to happen by literally 1 second would most definitely alter which exact sperm was used when conceiving both of his first two children.
Plot hole: In the unedited video footage of the helicopter incident shown to the crowd, the last shot of Arnie getting knocked out is seen from his perspective and as such could never have been filmed by any camera.
Continuity mistake: In the final scene on the bridge, Spock and McCoy are wearing colored arm bands which identify their departments: Spock's is orange (for Science) and McCoy's is green (for Medical). In the very last shot, Spock is wearing the green band and McCoy is wearing the orange one.
Revealing mistake: In the beginning, during the riot outbreak, there is a scene with a soldier being pulled down from climbing a fence. In the next shot as the crowd is pushing forward, you can see a concrete road block tip over. If you look closely at the bottom of the block, you can see that it's not a real concrete block, but a prop made of wood planks and grey painted rubber.
Other mistake: Towards the end of the movie, Carmen hands a guy some 3D glasses with a crack in one of the lenses and says something like "Sorry it's the last one," but less than a minute later hands a different guy a pair of 3D glasses.
Continuity mistake: When John is driving to work and he hears Anna on the radio, he swerves to avoid a car. From inside the car you can see that there are no cars in the rear window, yet when the shot changes to an outside shot, there are 3 cars. (00:33:35)
Continuity mistake: When Theodore goes to Amy for comfort in her apartment, there's someone (or something), walking at the front door, but then not again in the next shot. Keep your eyes top right, it's very visible. (01:26:50)
Continuity mistake: The head of Mega-Maid mysteriously shrinks in size. After Spaceball-1 transforms into Mega-Maid, the head is gigantic enough for a Winnebago to fly through for a considerable length of time. However, later on in the film, after the head has been jettisoned and crashed on a beach, the head is only big enough to hold a few dozen people. This can be determined by the size of the people coming out of her nose.
Continuity mistake: While this movie is so bad and full of mistakes, its barely worth finding them all, one stands out in the history of movie mistakes. One of the officers on board the ship, Lt. Lamont, is killed by the villain. A few scenes later she shows up on the bridge as an extra.
Continuity mistake: Carmen is wearing white sneakers riding with Juni on the magnetic transporter. After jumping off the magnetic rider and entering the hidden temple, Carmen is wearing black sneakers.
Continuity mistake: Toward the end, Carmen Ibanez gets stabbed through the shoulder when she is brought before the big master alien thing. If you were to sustain such an injury, there would be so much swelling, not to mention just destroyed tissue, that you wouldn't be able to move that arm for weeks. Yet a few minutes later, she is seen hefting around a very large assault rifle, and seems to have no difficulty. I guess it's a pretty standard action movie "heroes can temporarily ignore injury in order to save the day" kind of oversight, but it just seems exaggerated in this case, considering how severe the shoulder trauma looked. (01:48:15 - 01:53:40)
Continuity mistake: The blood stain on Kirk's jacket keeps moving around.