While the nurse is in the apartment with the old couple she locks the door with a chain. When she leaves, she unlocks the door and leaves the chain unlocked. Later when the mob tries to break down the door, it's locked and chained. 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 nurse escapes the French couple's flat and reaches the underground parking lot, blood is on her hands and leg. Soon after, her car crashes. When she climbs out of the front window with the doctor, there is no blood on her.
When Rollo is explaining to St. Luc what Hobbes was researching he tells him "You're gonna say it's crazy", and St. Luc's position changes from the previous shot even though the scene is supposed to look like one continuous shot.
When a guy crashes his car into St. Luc's in underground garage, St. Luc is in different position in the following shot which shown how the crash impacted his position.
When St. Luc is fighting an infected man in the underground garage, he knocks down some boxes behind him while he is swinging. They change position between shots and their position clearly shows discontinuity and numerous takes the scene probably took to film.
In the credits, the room described as the dentist's office is filled with optometry equipment.
The eel that Detective Haller keeps in his aquarium is actually a caecilian, a limbless, aquatic, nearly blind amphibian.
The manager cuts a building's phone lines. Later, Roger St. Luc rings the old francophone couple from the lobby after being attacked in the basement. The couple answer the phone and tell St. Luc that his girlfriend, the nurse, has left the apartment because the phone had been cut off. Roger used the intercom, not the phone lines.
When the parasite attacks Betts in the bathtub, it removes the drain plug and comes out of the drain. The water level remains the same, until the following scene when she gets out.
In the theatrical version, the wire that moves the parasite is visible in the garden, after it falls onto the old woman's umbrella. The wire is invisible in the home video editions.