Revealing mistake: When the chair moves across the kitchen floor, Craig T. Nelson goes to investigate it (after it has stopped moving). He goes up to the chair and flips it over. If you look closely, Craig almost hits the camera with the chair's legs. You can tell this has happened because the camera moves up very quickly and Craig looks straight at the camera crew with very big guilty eyes. This is more easily visible on the widescreen version of the film. (00:33:30)
Revealing mistake: When the oldest son is being pursued by the unseen entity whilst the family is seeing the priest. You can see a crew member in green pants and brown shoes to the left as the camera perspective turns upside down.
Continuity mistake: When the Swamp thing goes to destroy one of the trucks chasing Cable, the trucks used are chevy blazers but the one he wrecks is a International scout.
Continuity mistake: At one point in the film, the wingspan of Q is mentioned. Later on, its length is mentioned. The monster is twenty feet longer than its wingspan.
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)
Visible crew/equipment: In the scene where the staff member is cleaning up the research room with the animal cadavers there's a top-down shot of him at the communication panel. Directly in front of him there's a microphone visible that seems to be held by a filming crew member, as it moves out of sight after some seconds. (00:15:35)
Character mistake: During the surgery, one of the surgeons says to give the patient 25 cc's of cyatril. Then one says make it 35 cc's. 35 cc's (or 35 mL) is a full large syringe. The other doctor uses what looks like a 3 mL syringe and it's only partly filled. It seems the surgeons wanted 0.35 cc's.