Character mistake: The last time Doc tries the defibrillator on Norris, he uses it on the abdomen instead of on the chest. He also positions the pads incorrectly. They should be placed diagonal, with the heart in the middle, not horizontal.
Visible crew/equipment: As Rick and Chris get in the Volkswagen Bug after Shelly returns it, you can see three crew members in the backseat window reflection. (00:35:55)
Continuity mistake: At the start of the film, the dog is seen going from room to room looking for food. Just before he goes into Dana's room, someone can be seen through the crack in the door, moving from left to right.
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.
Audio problem: During the scene where Dr. Challis goes to visit his two kids, they put on their Halloween masks and his little son says "Let's watch TV." He goes to turn the knob to turn on the TV and a split second before he turns it, we can hear the intro of the commercial on the television.
Visible crew/equipment: When the PE coach arrives home, and walks inside, as she bends down to put her bag on the floor, you can see some equipment moving in the glass frame of the photo above her, on the shelf. (00:18:35)
Continuity mistake: When in the rowboat, Adrienne Barbeau's t-shirt is white in some shots and blue in others. It is not the lighting - it is plainly two different coloured t-shirts.
Revealing mistake: When the bugs crawl out of E.G. Marshall's mouth, it is obviously a dummy.
Continuity mistake: During the first rape scene, Carla is slapped in the face drawing blood and a pillow is placed over her face. After the scene, however, there is no blood on her face or the pillow.
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.
Revealing mistake: When Meg Tilly is being drawn toward the casket by the strange pink light the telekinetic dead man Karl Raymar emanates you can see strings on her feet pulling her toward it.
Revealing mistake: During the autopsy scene, it's obvious that the Panther is still breathing when it's supposed to be dead.
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.
Revealing mistake: Throughout the movie, practically every victim ends up with a broken, twisted neck. The "special effect" used for those deaths is...the actors wear their jackets backwards. It is made even more evident by the plot twist at the end.
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)