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During season two shooting, Elisha Cuthbert was bitten by the mountain lion who menaced her in one of the plotlines. Before shooting the scene the animal trainer suggested she "meet" the mountain lion before shooting, when she went to pet the mountain lion it bit her hand. She was taken to the hospital, treated for puncture wounds and was given a tetanus shot.

In season 3, it's revealed that Jack received an intricate tattoo on the inside of his left forearm as part of his previous deep-cover operation with the Salazar brothers. Instead of spending hours in the makeup chair on days when the tattoo needed to be shown, Kiefer Sutherland opted instead simply to get the actual tattoo.

In the fifth episode of the fourth season, Debbie's cell phone rang with a valid California phone number on the caller ID. Many fans noticed this and called the number, Some callers heard a voice mail message telling them that they called the 24 prop phone and to leave a message. Others spoke with various people, including Production Designer Joseph A. Hodges and stars Kim Raver (Audrey Raines) and Carlos Bernard (Tony Almeida). Hodges later stated in an interview that director Jon Cassar came up with the idea for the "Fan Phone" because the two of them often got bored scouting new shooting locations and wanted a chance to interact with the audience. Hodges indicated they received around 80,000 calls on the line in the first week.