Near the end, when George Stark is holding Thad (Timothy Hutton)'s twins, his gun is a Colt 1911 (as he's had throughout the movie). When Thad moves closer, the gun changes to a Beretta 92FS. After Thad takes one of the twins, George's gun is again a 1911 when he tucks it in his waistband.
Thad (Timothy Hutton) underlines the letter 'S' twice on the chalkboard, but in subsequent shots, there are more underlines.
The Toronado, a luxury car, was never offered with a 4-speed manual.
When Thad (Timothy Hutton) stabs himself in the hand with the pencil, he stabs his left hand. The next scene shows the pencil sticking out of his right hand.
(At 2:01:29) The 1911 locks open on an empty magazine. A second later, the 1911 is magically loaded again.
When Stark (Timothy Hutton) opens his straight razor to threaten Miriam (Rutanya Alda) in her apartment, the sound effect is that of a switchblade opening. A straight razor opening would make no sound at all.
George Stark, Thad Beaumont (Timothy Hutton)'s alter ego, was supposed to be from Oxford, Mississippi, which is in Lafayette County. The Mississippi license plate on his car incorrectly said Harrison County, which is on the coast of Mississippi at the opposite end of the state from Oxford.
The two babies were identical twins even though one was a girl and the other was a boy. This can happen in extremely rare circumstances due to a genetic mutation or an unusual fertilization scenario. However, the twins were played by identical twin girls.