Continuity mistake: When the boy stabs the man in the back with an ax while they're chopping firewood, the position of the ax changes between shots. In the first shot, the ax is vertical because the boy is behind the man. In the next shot, the ax is horizontal as if the boy is on the right side. (01:06:53)
Continuity mistake: Velma adds books to the pile in front of her when she takes the telephone order. Looking down the pile, you don't see any trace of the random loose papers that appear when she hauls the whole lot to the desk with the TV on. (00:06:25)
Character mistake: There are signs out front as the Spenglers arrive at the dirt farm. One of the signs is a call back to a scene in Ghostbusters 1984. In the scene, Ray quotes the book of Revelation: "And I looked, and he opened the sixth seal, and lo, there was a great earthquake. And the sun turned black as sack cloth, and the moon turned blood." When Finn Wolfhard reads it he misquotes the line saying "The sun became as black as cloth."
Continuity mistake: Radha Mitchell is giving a press release at the beginning of the movie. There are many microphones in front of her in rows. In the front row, from her left, are orange, black and blue microphones. In one shot from the front, another black microphone appears next to the blue one. (00:03:35)
Character mistake: When the 3 boys and Rogers are trying to recruit scouts, they pledge to Rogers and Carter raises his left hand but in the next shot he has his right hand raised. (00:09:05)
Continuity mistake: When Rory sees the porch door open by itself it is wide open. When he approaches the door, it is only ajar. (00:21:15)
Continuity mistake: Nicholas Cage sees his ghostly son through the window of a bus, and so he rushes after him. Right at the first left turn, the natural light couldn't be any more different in the two separate shots. (00:32:30)
Continuity mistake: When Jamie gets out of her car, her leg warmers reach down to her heels. In the next shot, they've been pulled up to just her ankles. (00:07:05)
Visible crew/equipment: When Dmitri gets weapons from the back of the government vehicle: you can see the reflection of 3 crew members in the window of the truck. In the same shot you can see a crew member's gloved hand holding a flashlight inside the vehicle to light the scene. (01:16:00)
Continuity mistake: As the police are seen prepping for battle, and Eigerman is delivering the line "Whether it be freaks, commies or third-world y-chromosome mutants we are there - the sons of the free", watch his hands as he picks up the large gun. As the camera cuts during the dialog, his hands have suddenly swapped positions on the gun.
Continuity mistake: Malorie pulls out her Sig Sauer P229 handgun and begins firing but the insane guy disarms her and the handgun falls. Malorie is on a boat and the crazy guy is standing in deep water. It isn't shown where the handgun lands when it falls out of the insane guy's hand. Later on Malorie pulls another Sig Sauer handgun from the same holster, however this time it's a model P228 or possibly P226, the 226 has a 0.5 inch longer barrel. The 229 and 228 have completely different slides. The 229 is a legacy. (00:36:50 - 01:01:30)
Continuity mistake: When the cannibal nurse is caught by the doctor, their poses change between cuts. (00:14:38)
Continuity mistake: When the Grand Witch throws the lectern, it lands 6 meters away from the chairs. When the shot changes, it's barely 2 meters away (note the wooden planks on the floor).
Continuity mistake: Towards the end, right after Mia crawls through the tunnel and into the shed, she backs up against some shelves. There is a lot of blood on her face and her hair is hanging over her face. The shot changes to a close-up and her face now has much less blood on it and the hair that was hanging in her face is gone. (01:16:25)
Continuity mistake: After Chloe runs off into the woods and the other two girls finally find her after she's had her run in with Slender Man, they are shining the flashlight in her eyes and asking her questions. In one shot her eyes are still open as she gazes near blankly off in the distance. When it cuts to a close up and suddenly her eyes are tightly squinted shut. (00:38:45)
Continuity mistake: When it shows Mandy reading from her book, she has nothing visible sticking up from it in the wide angle. When it cuts to a close up suddenly the dollar bill she's using as a bookmark appears. It's first on the right side, then switches to the left side and further down. (00:17:30)
Revealing mistake: When Austin goes to write the Roman numeral "XVII" on the chalkboard, the erased numerals from a previous take are still visible where he's about to write it. (00:41:36)
Factual error: There is a calendar in a scene towards the end of the movie which shows the date as Wednesday April 30, 2021 but the calendar does not align to the days of a real calendar. That date will be a Friday. (01:25:54)
Visible crew/equipment: When Skip has just finished snorting cocaine and is talking to Stella, the boom microphone is visible on the top left of the screen.
Continuity mistake: When Amy says that she lost her earring, she's wearing shades. The shot cuts to Stacy and then back to Amy, who is now wearing spectacles without having had enough time to change her glasses off screen. (00:03:40)