In 1997, Vince Gilligan was tasked with writing the 12th episode of season 5, to be shot after the show went on a few week break for the holidays. His idea was to have The X-Files crossover with Unsolved Mysteries, and on paper, it seems inspired. The story would've seen a standard X-Files case of the week profiled as if it was a segment on Unsolved Mysteries, complete with host Robert Stack narrating the action. An added comedic element involved different actors playing Mulder and Scully in the show's reenactments, which stars David Duchovny and Gillian Anderson were apparently quite on board with, as it would've required a lot less work on their parts.
Gilligan hit a creative wall when it came to actually writing the full crossover episode. He just couldn't figure out how to make it work as a whole, and eventually just shelved the idea. This left him still needing to come up with a new script, which thankfully ended up being the fan-favorite X-Files episode "Bad Blood."
Via Screen Rant