Plot hole: Krell doorways and equipment suggest they are short, wide and 2-3 times larger than a human. Yet the shuttle pod the Commander, Doctor and Morbius ride in is designed for human size beings.
Plot hole: During the Congressional hearing, it is suggested that the alien signal could have been faked from a satellite - as Ellie only has her own experience to go on, it leaves her believing what happened, but still with an element of doubt. However, as a professional astronomer, Ellie would have immediately dismissed this since a simple parallax (triangulation) would have confirmed that source was at the distance of Vega - a distance far too great for any rocket to reach. (This is mentioned in the book). In fact, this is mentioned early in the film, when the employees in New Mexico calculate the source of the signal while it is transmitting prime numbers.
Plot hole: The whole plot of this movie stems from a scientist selling his time machine to a corporation for further funding for development. He has a TIME MACHINE. Lotteries, stock market, 100 years of interest on a savings account. Why does he need more funding?
Plot hole: A nuclear armed stealth fighter is accidentally sent back in time to 1940's Nazi Germany. After the Nazis capture it, two days later they drop a nuclear bomb on the United States. There are several basic problems here. Firstly, if the Nazis captured advanced technology, wouldn't they spend more time taking the plane apart and interrogating the captured pilot? How on earth did they manage to train a pilot to fly the stealth in such a short length of time? And how did the stealth fighter manage to reach the U.S. from Germany when in-flight refuelling was unknown then?
Plot hole: The assassin cuts a hole in the glass to kill the Federation president. But with the size of the hole and the placement of the scope on the phaser rifle, the assassin would not have been able to see out of the hole.
Plot hole: The super warriors that are given the venom are supposed to be fearless and without self preservation. However, during the scene in the Sahara base when they are helping to get the warheads back from the Joes, one of those men screams out in fear as he falls down the shaft, which makes no sense as the one guy did not scream in frustration when he was bitten earlier by the deadly snake and knew he would die. (00:40:20)
Plot hole: When Ripley goes to access the computer Mother, the entry door opens and closes with a hissing noise, a couple of minutes later you see Ash next to Ripley with a smile on his face. How did Ash manage to open the same door without it making the hissing sound? Ash could not have been in the room before Ripley came in - once Ripley has sat down at the console, it rotates on a 90 degree angle, this would have made it impossible for Ash to be hiding behind a desk or a computer console without being seen by Ripley. There are many doors on the ship that make the same hissing sound when the doors open, an obvious example is the infirmary door. And Ash couldn't have disabled the pneumatics of the door, because when Ripley exits the room, the hissing noise is heard again. (01:18:40 - 01:20:00)
Suggested correction: There's no evidence that sound happens every time the door opens. It may be a pressure release sound, but not enough pressure has built up since Ripley opened the door to make the sound. It's like flushing a toilet twice in quick succession, before the tank has had a chance to refill. The action is the same, but the same mechanism is engaged; it makes a different sound.
Plot hole: When Jack The Ripper checks his watch before confronting H.G. Wells and demanding the key, the hands point to 8:50, the ensuing chase to the museum and demise of the Ripper may have used up a half hour or so, yet the time on the wall clock indicates that it is midnight.
Suggested correction: Movie time and real time don't match, so 3 hours has passed without all 3 hours being shown. The fact that the clock now shows midnight is meant to explain this fact without the need for subtitles to reveal the time.
Plot hole: Brent discovers that Taylor is on the planet and has a connection to Nova through the dog tags she is wearing. However, throughout the first movie, Taylor never had any dog tags on his person. Also, his ship was destroyed, and he only managed to retrieve a life raft and supplies. His belongings and clothing were taken from him when he was apprehended by the apes. Taylor should never have had any dog tags to give to Nova or for Brent to discover. (00:11:10)
Plot hole: When Gwildor tells Julie and Kevin that he can use the Cosmic Key to take them to any point in time they wish, Kevin asks just for them to go straight home. As Kevin and Julie go through the portal, Julie turns around and shouts to Gwildor to send them back before being dragged through the portal completely. When Julie wakes up in bed, she discovers that Gwildor has sent her back to the day her parents were killed. Julie never said where in time she wanted to go, so there's no way Gwildor could know to send her back to the day her parents died.
Plot hole: When the aliens kidnap the adults, the kids are left behind. But all the kids seen are around 11 and younger. Where are the teenagers or young adults? None of them are during the celebration scene nor in the arena on the alien planet.
Plot hole: When Becky falls asleep she is awakened by Miles and has now become an alien, However she should have died and her essence should have been in one of the pods. There were none nearby.
Plot hole: The movie is based in 1986 after the group time travels from 2010. That makes it a 24 year trip back in time. It is stated that Jacob is 20 years old. That is an error as this would make him roughly around 23 or 24 if he was conceived in 1986, which we saw near the end of the movie.
Suggested correction: This mistake has already been corrected. Cusack could have simply generalized his age when he mentioned it, probably not remembering his real age at the moment.
Plot hole: Everybody jumps into the water and emerges on the submarine except for Natalie. They're wondering where she is when they notice a flare. They realise it's her, and they send a search party. She's hiding on a bus maybe one to two kilometres away from the water. Her going there makes no plausible sense. Makes no sense how she held her breath underwater longer than anybody that they didn't see her and why she would go in the opposite direction of the sub. It was done to prolong the scene. (01:09:00 - 01:18:00)
Plot hole: Throughout the film, anybody who speaks English uses Modern English, with either a British or French accent as appropriate. However, 1357 was solidly in the Middle English period. Although that language would not be utterly incomprehensible to modern-day travellers, neither would it be indistinguishable as it was in the film. The film specifically draws the viewer's attention to language at several points, making suspension of disbelief impossible. The French speak French, and very few of them speak English. There is a scene in which Andre is talking to Claire, and her lack of knowledge of modern idioms makes the conversation difficult; that should be true for all characters at all times. If the historical people were talking alone among themselves, I could accept that they're being "translated" for us. But they're talking directly to the modern travellers - it's too jarring.
Plot hole: When the team plays back the record, it contains an extremely high-pitched tone which triggers the crystal sensors on the ship. But the speech on the record is low quality. This low quality would be the result of extreme frequencies being lost; but if they were, the trigger tone (a VERY extreme frequency) would be lost too. If the League's gramophones are capable of recording and playing such a tone, they should also be able to record and play back the speech without any quality loss. (01:09:05)
Plot hole: Michael McKean quits pursuing Beldar and approximately 15 years pass as indicated by Connie growing up, yet when you see McKean again and his assistant, they have not aged at all.
Plot hole: When Martin decides to antagonize David by eating spinach, he eats it from his plate. When they both eat the spinach frantically, they each eat out of a large bowl. Where did the second large bowl come from, and why would there even be a second one if they only need enough for three people? (00:34:55)
Plot hole: One of the flying dragons impacts the right outer engine of the B-29, and the fire spreads to the right inner engine. The crew lands the plane on the island, and the girls rebuild both engines, despite the fact there are no parts available on the island.