Incorrectly regarded as goof: "Roger St. Luc rings the old Québequois couple from the lobby after being attacked in the basement. In the previous scene, however, the viewer sees the manager cut the phone lines. Despite this, the French couple answers the phone and tell St. Luc that his girlfriend, the nurse, has left the apartment. But the reason that she left the apartment was because the phone had been cut off. "
Roger calls using the intercom system, not the phone lines.
When the nurse escapes the French couple's flat and reaches the underground car park, she has blood over her hands and leg. However shortly after her car is crashed and she climbs out of the front window with the doctor there is no blood on her.
While the nurse is in the apartment with the old couple she locks the door with a chain, but after-wards she leaves the apartment (leaving the chain out and unlocks the door). Later in the movie when the mob tries to break in the door it's firstly locked and then the chain is also visible holding the door closed. The old couple is still sitting in the couch in the same position which indicates they never manually re-locked the door or put up the chain a second time.
When the parasite attacks Betts in the bathtub, it removes the drain plug and itself comes out of the drain; however, the water in the bathtub never starts draining. The water level remains the same until the following scene when she gets out.
In the theatrical version, the viewer can see the wire that moves the parasite in the very first shot it appears in (in the garden, after it fell on the old woman's umbrella). However, the wire is so thin that it is invisible in all home video editions.