One of the most interesting cancelled films i've heard about in recent years is a cancelled direct-to-video movie for the TV show "Recess", it was announced in a Disney press release and was supposed to come out on August 17th 2004 but never did and there was never any mention of the film ever again. So far basically no other info is known about the film(no cast list, cover art, plot synopsis etc) beyond what is on this wiki:First Day of School
We don't even know how much progress was made on the film. I do have to wonder if Michael Eisner resigning from Disney played any role in us not hearing about this film again(as I know Dumbo 2 was planned and announced and never happened due to Bob Iger wanting to "restore Disney" by cancelling all the Disney DTV sequels that weren't already mostly complete, so perhaps this movie was quietly cancelled as a result).
I'd love to get to the bottom of this mysterious film.