Near the end, when George Stark is holding Thad'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 underlines the letter S twice on the chalkboard, but in subsequent shots there are more underlines.
In the scene where Thad is writing in his office with the sparrows on the window, the sharpness of the pencil he uses changes back and forth from very sharp to very dull. This is the scene where Thad stabs himself in the hand.
The Toronado, a luxury car, was never offered with a 4-speed manual.
When Thad 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. At 2:01:30 the 1911 is magically loaded again.
When Stark opens his straight razor to threaten Marian in her apartment, the sound effect is that of a switch blade opening. A straight razor opening would make no sound at all.
George Stark, Thad Beaumont'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 female and one was a male. Played by identical twin girls.