Plot hole: Patrick Redfern needs someone to be in the boat with him to corroborate his and his wife's deception and be his alibi. That is whole point of his wife lying on beach - he gets this when taking Myra for a ride - but it was portrayed as a random ride she got, he did not intend or plan giving her a ride, he did not see her loitering near the boat, or even in view of him untying the boat. Even if that was intention in the story for him to be seen ready to cast off, the movie viewers did not see her viewing him, only her appearing just before he was set to cast off. Without someone with him the whole deception would not work, he would have no alibi, he could have killed Arlena unseen.
Plot hole: It's never explicitly stated or shown that the Thing reproduces with each victim until the movie is nearly over (when Palmer infects Windows). Most viewers figure it out from the context, but it's unclear just when and how the characters themselves have come to this conclusion. This was an inadvertent result of an editing decision and a visual goof: there is a deleted scene in which Blair explains much more directly that the Thing multiplies according to how many victims it takes, and in its place in the final film is a scene containing a computer simulation that director John Carpenter acknowledges was a failed attempt at explaining the organism's life cycle.
Plot hole: Why doesn't Reliant know that Khan is exiled here? The Federation is so terrified and opposed to genetic engineering that it's still illegal 300 years after Khan. So why is there no warning along with the data on the Ceti Alpha system? Kirk logged what happened with Khan and his solution of marooning him. Starships use nav data to navigate star systems. Ceti Alpha 6 exploded, yet the helmsman or computer never noticed that there is one less planet than there was when Kirk was there? There is no debris from the explosion? Ceti Alpha 5 is the exact same size and was conveniently blown into the exact same orbit as Ceti Alpha 6 used to have? So there is nothing whatsoever to make the crew even suspect that it's not 6? Enterprise would have to have scanned the planets in the system to know that one was habitable for Khan. Did Ceti Alpha 6's destruction somehow magically turn Ceti Alpha 5 into its exact duplicate? If Starfleet ships have been there to map after Ceti Alpha 6 exploded, none of them bothered to check on the exiles? Pretty callous for Starfleet, don't you think? With the technology and amounts of information available to Starfleet vessels, there is NO logical reason for the Reliant to think that this planet is Ceti Alpha 6. Finally, would the Federation be willing to test a device whose exact effects will be unknown on a planet so close to another inhabited one? (00:21:00)
Suggested correction: The answer is yes: against all known laws of science, the inexplicable explosion of Ceti Alpha VI led to Ceti Alpha V conveniently taking its orbit and making it easy to mistake for its former sister planet. A mistake would've been to give an explanation that can be debunked. By leaving it to "somehow" the movie leaves it open to a million possible rationalizations. You can even make a whole other story about the crazy circumstances that led to this incredible result.
Plot hole: When Rambo hijacks the Army truck, he throws out the driver, Robert A. and continues driving down the one-lane dirt road in the same direction. When Robert A. hits the ground, he immediately pops up and runs the opposite direction, away from where the truck is driving. A couple of shots later, we see a behind view of Robert A. running. It turns out that it is the driver's view from a Sheriff's vehicle. How could a car, especially a Sheriff's squad car, possibly come from that direction since it was a one-way road? And how could that same officer not see that the man driving the truck was Rambo?
Plot hole: We learn in the film that Cat People must kill a human in order to return to their human form. However, after becoming a cat at the swimming pool and terrorising Alice, Irena becomes human again without making a kill (the only other person at the closed pool is alive and well at the end of the scene - we know Alice has not imagined the presence of the cat because of her shredded clothes). (01:31:00)