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Props from Mulder's X-Files office are currently preserved and on display at the Hollywood Entertainment Museum in Los Angeles. According to the museum, the famous I Want to Believe UFO poster from the office continually had to be replaced as copies kept disappearing from the set. The poster on display at the museum is reportedly one of the last available copies of the original set-used posters.
The famous echoing chord from the theme music was a fluke. Composer Mark Snow accidentally rested his elbow on his keyboard with the echo function on and he liked the resulting sound so much, he wrote the theme around it.
When Gillian Anderson got pregnant at the start of the first season, some network executives wanted her role re-cast. Executive producer Chris Carter adamantly refused their request.
In the early years of the series, Mulder is a firm believer in extraterrestrials and Scully is skeptical. In real life, this is the opposite, David Duchovny being the skeptic and Gillian Anderson the believer.
Chris Carter wanted the show to end after the fifth season, when his original contract, as well as those of Gillian Anderson and David Duchovny expired. Carter wanted to continue the story in a series of feature films, starting with The X Files (1998). The FOX Network, however, found demand for the show too high to cancel the series, so they instead mandated the film to fit in between seasons five and six of the show. Carter found writing seasons five and six as well as the film very difficult, as he had to craft a screenplay that would not require the audience to have seen the show to understand it, and episodes of the series that would tie in to the film, but not rely directly on the film's plot to make sense.
David Duchovny and Gillian Anderson attended the Golden Globes ceremony together in 1997 and were holding hands the whole night. Between them they won best lead actor and actress in a drama series. Anderson could be seen cursing backstage right after picking up her award while watching Duchovny thanking her when he picked up his; she had just realized that she had forgotten to thank him.