Sarah Clarke was cast as Nina Myers on the morning filming for the pilot episode was to begin. The costume department did not have time to fit her, so she wore her own outfit for the entire season.
The main shot of the 747 exploding was removed, the show had its premiere delayed a month and this scene cut as well as further storyline changes, due to the attacks on the World Trade Center. Nearly all later references to the plane explosion were also removed.
Season 1 is the only season of 24 to actually take place on 1 continuous day (midnight to midnight). All subsequent season were 24 hours but took place on two separate days.
Richard Burgi was originally cast as CTU director Jack Bauer until Kiefer Sutherland showed up. Once Sutherland took the role, he was recast as Alan York in Day 1.
After the 9/11 terrorist attacks, it was uncertain whether the series would be able to air for the 2001-02 TV season. However, the only change made to the series involved a sequence in the pilot where a villain played by Mia Kirshner armed a bomb on a passenger jet and then parachuted away as the plane exploded in the background. This sequence ended with her character killing an agent and then preparing to escape, with the actual escape and explosion cut from the broadcast.