Plot hole: Frank Ochoa told miss Nicole that Paul killed 9 people in New York when in fact he killed 10. (00:47:00)
Factual error: At the beginning all the children sleep in the same room on Christmas night. On the wall in the room, next to the door, hangs a string with five different kinds of small cross flags, including the Swedish and Norwegian flags, which are correct for the era, but also a white flag with a blue cross on the string, even though this is almost a decade before Finland would declare independence.
Continuity mistake: Myra Bruhl is popping pills at the beginning of the movie; when she puts down the container, there's a white-labeled bottle on the bedstand that was not there before. (00:02:40)
Revealing mistake: Chong arrives at the curbside of the airport. The guy pulls up in a yellow Datsun for him. The fender is already dented. Chong starts throwing the guy into the fender at the exact point of the dent - the fender must've gotten dented in an earlier take.
Character mistake: Thérèse mentioned that since 6:00pm, there was one call. After the second call from the man who commited suicide at a pay phone, there was two calls since 6:00pm. But Pierre said: "UN seul appel depuis 18h..." (ONE call since 6:00pm...) instead of "two calls."
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.
Continuity mistake: In the prologue, the boss of the bad guys approaches the window. Notice the tree casting a large shadow. He whispers "Jessie, try the door", and in this close-up there's a lot of light and it's his hand and head to cast a shadow on the wall. (00:03:15)
Continuity mistake: In the scene just before the snowman comes to life, the clock in the hallway reads midnight. But a little later, when he is showing the snowman around the house and he is about to lead him upstairs, the clock reads 8:05.
Continuity mistake: John is helping Mr. Nardini at the beginning of the movie. The box breaks right away and spills its content on the doorsteps, and the various items (pay attention to the blue bags of pasta for instance) shift position at every cut. (00:03:50)
Continuity mistake: Near the beginning when the guy is robbing the liquor store, he's holding his gun with one hand. In the next shot, he's holding it with two hands.
Visible crew/equipment: When the plane drops the grenade into Halicki's car, and he throws it through the sunroof, it blows up the car tailing him. In the slow motion shot following, the stuntman's crash helmet is visible. (00:50:00)
Continuity mistake: When "Bruce Lee" is fighting the bad guys after he catches the poker cheater, there are multiple continuity mistakes; the bad guys are knocked atop the tables in the room but they disappear and reappear from those tables between shots.
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.