This is the 100th episode.
Richard Belzer crosses over as Detective John Munch, the character he played for seven seasons on the series Homicide: Life on the Street (1993) as well as four episodes of Law & Order (1990) plus the TV movie Homicide: The Movie (2000). When Homicide: Life on the Street (1993) ended in 1999, he went on to play the same role on Law & Order: Special Victims Unit (1999) for fifteen seasons. He also cropped up as that character in individual episodes of The Beat (2000), Law & Order: Trial by Jury (2005), Arrested Development (2003), The Wire (2002), Jimmy Kimmel Live! (2003), 30 Rock (2006), and 30 Rock (2006) (in an uncredited role). Consequently, he is the only actor in television history to play the same character on eleven different series.
This episode - a Lone Gunmen centric one - came about because David Duchovny and Gillian Anderson were still involved with shooting The X Files (1998).
Normally during the filming of an episode for the show, Bruce Harwood, Tom Braidwood and Dean Haglund would share a trailer. Because this episode was all about the Lone Gunmen, they were offered separate trailers. They declined.