John Doe's second daughter was born on the first day of filming.
David Carradine was originally supposed to reappear towards the end, but he walked off the movie before the original intended ending was shot, and hence the ending had to be re-written without him.
Frederic Forrest and Max Perlich were originally considered to play the leads.
Adam Horovitz's Character says he was born in the late 70's when discussing a 60's based topic. Horovitz was actually born in 1966, and was roughly 25 years old around filming, and could not portray a teenager.