Revealing mistake: When Krampus walks toward Max after he yells at him to give him his family back, wires are controlling Krampus' arm. (01:24:30)
Revealing mistake: Just before Madison is about to be pulled up the stairs, it's quite too obvious that is isn't Kennedi Clements, rather a stunt double.
Revealing mistake: During the "fognado," when Fin is lifted off the ground, his shirt is pulled up due to the harness and its cables.
Revealing mistake: When Mike browses his own everyday phone looking at the photos, the phone shows a "no SIM" message. (01:00:00)
Revealing mistake: The idea is that the characters are in some super remote area where there are no working phones and nowhere to get help, but throughout the movie you can see a town in the background with buildings.
Revealing mistake: From the sound made when the car hit the deer, as well as the deer's overall appearance, it was obvious the deer was fake. The fake deer's body was almost uniformly brown, but the "dead deer" on the road had a white underbody that went part way up its side and along its legs, which should have been visible on the "fake deer." (00:10:19)