When John is trying to warn everyone to get off the bridge at the end, he has gloves on in some shots, and bare hands in some others.
When John is driving on the freeway we see the lights of headlights on his face, but other shots of the car show no cars coming towards him.
After John's car stalls, he gets out of his car and looks down at his shoes. He is wearing dark brown or black, glossy dress shoes. A few minutes later, when he is standing in Gordon's shower stall, John is wearing light-brown, Nubuck casual shoes.
At around 29 minutes, Mr Smallwood walks into the paint store followed by another guy but when the camera pans back around the second guy is opening the door again to go in the store.
As the characters are standing outside the police station, four of the extras (one of whom is a teacher called Mrs. Mickola, as it happens) walks past in the background twice from the same direction.
46 people actually died in reality, not 36 like the movie claimed.
The Washington, DC license plates on John's car start with S - in 2018, the highest letter in the alphabet for these plates is an F.
In the scene where John Klein is talking on the phone explaining that he didn't leave the voice mail about the chemical plant, he walks to the mirror and the mirror doesn't match what he was doing. His body movements were a few seconds ahead of the image in the mirror. (This may have been intentional due to the paranormal nature of the film, but they may also have had to add in the mirror image later because the camera was visible and not synchronized it properly.)
When John Klein rips the telephone from the wall and throws it, we hear the sound of a telephone with a bell hitting the floor, yet the phone is a modern digital cordless phone.
An obvious Pittsburgh landmark (the Masonic Temple, home of the Pitt Alumni Association) appears in the background of scenes supposedly set in Washington, DC, and Chicago.
During the Christmas tree lighting festivities, the door of the fire truck reads "Saxonburg" (a town near Kittanning in Pennsylvania where the scene was filmed) not Point Pleasant (where it is set).
On Klein's drive from Point Pleasant to Chicago, a glimpse of an exit sign for "New Kensington" can be seen. There is no "New Kensington" between West Virginia and Illinois, but there is one near Pittsburgh, where the movie was filmed.
Twice Point Pleasant, West Virginia, is mentioned as being adjacent to Indiana. But Ohio is between West Virginia and Indiana.
John passes a sign for Maine Avenue as he leaves Washington, driving from Memorial Bridge to Interstate 95. Maine Ave is in the opposite direction and there are no signs for Maine Avenue on any of the roads he could have taken to get to I95.
In the scene with the Washington Post reporter, John says that the doctors operated on her temporal lobe for the cancer, but the x-ray showed the tumor in the frontal lobe.
When John consults the supernatural expert/book author Albert Leek in the library, Leek refers to other times Mothmen have been observed before big disasters. He mentions the Galveston Texas hurricane of "1969" yet alludes to the big one that happened in 1900 and killed about 6000 people.
John Klein and several medical professionals mispronounce "glioblastoma" as "gly-o-blastoma" when it should be pronounced as "glee-o-blastoma."
Albert is incorrect. The character's name is Alexander Leek.