 The final episode, "Sleeping in Light", which aired at the end of  the fifth and final season, was actually filmed at the end of the  fourth season. At the time that production on the fourth season  wrapped, it was still unclear whether a fifth season would be  produced or if the show would be canceled. When the show was  renewed, they filled the last slot in the fourth season with a new  episode, and showed Sleeping in Light at the end of the series. This  was practical because the events of Sleeping in Light take place 20  years after the other episodes.