Tonight's Simpsons episode, The Man Who Came To Be Dinner, features the family getting launched into space and meeting Kang & Kodos in their first major appearance in a non-Halloween, non-clip show context.
The episode has had a long, strange journey: it was first announced back in September 2012 and scheduled to air in May 2013 as the Season 24 finale, but it was mysteriously postponed just two weeks before it was supposed to air. It also did not air in the following season as expected, but is now finally airing in the middle of the current season, nearly 2½ years after it was announced.
Fans had a number of theories about the delay: Animation problems? A lawsuit from Disney? Cold feet about the out-there premise? Saving it for the series finale?
Producers Al Jean and David Mirkin finally revealed the real reason on Twitter: at some point they seriously considered scrapping the episode and reworking it into a sequel to The Simpsons Movie.