Continuity mistake: Hatchet marks are already visible on the door when Vic Nottingham arrives to chop through it.
Continuity mistake: It was briefly stated in the second film that the T-Virus was created by Dr. Ashford to cure his daughter Angela and that the Red and White Queens were based on her. This movie contradicts that by suggesting that Dr. Marcus created the T-Virus to cure his daughter Alicia, and that the Red/White Queens were modeled after her. While the rest of the movie stays true to the continuity of the previous films, this rather obvious ret-con is still a major continuity gaff.
Continuity mistake: He cuts a hole in her pants for insemination. When in the car moments before fighting the dog there is no hole.
Continuity mistake: When the Senator is about to be sacrificed, the cloth on her mouth immediately disappears for a split second. (01:29:35)
Visible crew/equipment: In the second scene where the main character approaches the car, there's the filming crew visible in the right corner of the car's bumper. (00:02:30)
Continuity mistake: When Maddie is coming up with an ending to her novel, she opens one of the ending drafts. Not long after opening the draft, she decides to go on facetime and make a call, whilst the draft is still open, then the man she is trying to contact calls her back. She then closes the laptop and tidies up. When she returns and opens up the laptop, the draft that was just shown on the laptop has become a completely different one. (00:10:10 - 00:14:25)
Factual error: The license plate of the car that takes the protagonist to the sanitarium is GR 36E46. That's not a valid Swiss plate: GR indicates the canton of Graubünden, but other than that it is supposed to have only digits and no letters.
Plot hole: Sang-hwa forces a zombie inside a toilet stall. The zombie begins to bang on the door violently and Sang-hwa has to hold onto the door in order to stop the zombie from getting out. However, it is revealed later on that the zombies do not attempt to attack the humans or react violently or aggressively if they cannot see them. If this is the case, then this scene of the zombie banging on the door even when it cannot see any of the uninfected humans contradicts the rules that are established later on. (00:25:48)
Continuity mistake: When Nancy is surfboarding, she is going to ride her last wave for the day. When she is waiting for her last wave, the ocean appears to be calm, but when it cuts to the very far 3rd person view a big wave pops up on the screen. When it cuts back the wave has disappeared.
Visible crew/equipment: When Marcy tries to shoot the dog, you can see a crew member who picks him up at the left side of the little appliance room.
Other mistake: When the Sheriff arrives at the end, his car has Pennsylvania license plates but we have been told that the setting is Virginia.
Revealing mistake: As Joe is gesturing, the wrinkles of the gloves worn to make his hands bigger are noticeable.
Continuity mistake: Throughout the movie, fire axes seem to be the weapons of choice. Only two fire axes are used, but they appear recurrently. In the very first use, the protagonists grab fire axes to butcher the monster called Beverly. Blood and gore go everywhere; yet, the axes emerge from this bloodbath with no blood or gore on the handles - they're virtually pristine moments later. The axes are used again and again, right up to the ending, and blood comes and goes from the axe handles seemingly at random.
Revealing mistake: When Gabby and Fin are hanging off the building in Las Vegas, you can see that Gabby isn't holding on to anything in some close-up shots. (00:11:47)
Continuity mistake: Mikey dropped the wristwatch Andrew gave him and it landed face down with the bands upward. When Mikey jumped off the rock to retrieve it, the watch's face was on its side (visible to the screen) and the bands were parallel to the ground. (00:05:04)
Continuity mistake: The cursed necklace is shown emitting its evil fumes on the white paper of the letter, but the letter itself is perfectly 90° with the envelope, or at a different angle. (00:06:20)
Revealing mistake: Right after the little girl sprays the monster with bug spray, you can see the costume of the monster, it's wearing a black cap. (01:19:00)
Audio problem: The chef finally gets the bright idea to fire his hunting rifle at Morgan's fleeing vehicle. In a previous scene he was grabbing ammunition for his rifle, and he also loaded it. The rounds appeared to be high power rifle ammo, probably in the 7mm Remington magnum or .30-06 range, very common hunting rounds. When he fires his rifle it sounds quieter than a suppressed shot, however he has no suppressor on the muzzle end of his weapon. (01:09:20)