To match the direction of movement, a shot of the Ford LTD racing around a corner has been flopped, but the license plate, which is now in reverse, is visible.
The fortuneteller scene at the party switches between Jacob's left and right hands.
Even through Jacob doesn't finish shaving (he towels-off the remaining shaving cream off his face), he appears to be clean and evenly shaven.
When Jacob and Geary are walking down the stairs, Geary is holding his briefcase with his left hand. After he says "war games in Thailand" and makes a motion with his right hand, the next shot shows that hand now holding the brief case.
When Jacob's kidnapped and the car makes a turn, the shot is flipped - the car's plate is mirrored.
The subway train Singer gets off of is marked as the "C" train, heading out to Rockaway Park. It never served the Bergen St. Station. However, the original script called for the station to be the Nostrand Ave. Station, which would have been factually correct. Also conspicuously absent from the train is graffiti, common to the subways of that era.
At the beginning of the film Tim Robbins' character is shown carrying an M-16A1 rifle - the forward assist being clearly visible on the right-Hanna side to the front of the trigger. The M-16s used In Vietnam didn't have forward assists so they were prone to jamming. The forward assist was added later to help alleviate the jamming problem.
A support wire is clearly visible on one of the demon's wings during the party scene.
One of the occult books Jacob is examining ("A Witch's Bible Volume 1") was not published until 1984, more than a decade after the movie is supposed to have taken place.
When Jake puts his son to bed, there is a crew member visible in the mirror behind him when he leans down.
Numerous times, most noticeable in a mirror.
Louis shouts out for "Jacob Stinger" at one point when he is demanding Jacob's release from hospital.