All of the prison scenes were filmed in a real prison (Washington State) with real prisoners.
During one of Evan's flashback scenes, he can be heard reading part of Ray Bradbury's short story "A Sound of Thunder." In this story, a group of people travel millions of years into the past to hunt dinosaurs. One of them accidentally steps on and kills a butterfly, which dramatically alters the future.
The lights flicker in the background of the psychiatrist's office when Evan is being hypnotized, after they blew up the mailbox. This was not a planned special effect - it was an actual short in the wiring on set. The directors thought it fit well with the scene and used that take in the final cut of the movie.
Was one of the most widely read unproduced scripts in the industry. It wasn't until writers/directors Eric Bress and J. Mackye Gruber agreed to write Final Destination 2 (2003) and Ashton Kutcher signed on as an executive producer of The Butterfly Effect that it was greenlit.
In early versions of the script, the character of Evan was originally Chris Treborn. When the "T" is moved over, it becomes "Christ Reborn". This was changed to Evan Treborn, which is a play on "Event Reborn".