I am always surprised how so many movies and expensive productions can be buried. It's a like hiding an office building that is 80% done. There are countless numbers of movies that have never made it to the light of day.
Citizen Kane - The granddaddy of all of the buried movies was Citizen Kane. By holding billionaire William Randolph Hearst in ridicule, Orson Welles drew fire on his first movie. Hearst papers ignored Citizen Kane and buried it. While it did win some praise at the Academy Awards, it disappeared until the 1950s when film aficionados uncovered the film and it was soon considered the best film ever made.
The Day the Clown Cried - The Holocaust is a touchy subject. For the sake of winning an Oscar, Jerry Lewis decided to make it the subject of his 1972 comedy about a clown at Auschwitz. It made it all the way to the point of screeners before being circulated. The movie was summarily buried. It was said that the prints and negatives were destroyed but rumors abound that copies stayed in circulation into the 1990s. Someone in Sweden held onto their copy, eventually transferred it to video and then to the Interwebs (http://isohunt.com/torrent_details/38423236/The+Day+the+Clown+Cried?tab=comments). There are very few seeds, so getting this via Bittorrent will only be slightly easier than building a time machine.
Hamster of Happiness / Secondhand Hearts - Robert Blake (murderer / Lil Rascal / actor) kept going onto the Johnny Carson show and talk about the Hamster of Happiness. Three years after production wrapped, this regrettable film saw the light as "Secondhand Hearts."
Fantastic Four - Roger Corman was a prolific producer. Someone he snagged the rights to Marvel's first superhero team, the Fantastic Four for his 1994 movie. My guess this happened in the era when superhero movies were low budget schlock fests and the rights would be easy to obtain. I got to watch this back in the 1990s. Supposedly, the film was poorly received so the studio bought the distribution rights, buried it and pledged to make a better version. The real sin of it: this movie is better than the two cinematic disasters that came out in the last couple years. Via Bittorrent, this movie lives on: http://isohunt.com/torrent_details/93028921/Roger+Corman+Fantastic+Four?tab=summary
I Love Lucy (the movie) - While it seems like the trend is to turn every lame TV series into a movie. This happened in the 1950s too. For fear it would distract from the other Lucy Ball / Desi Arnez comedy, The Long Trailer, the studio buried it. The movie will see the light of day on April 27th 2010, as a DVD release. Ricky!!!
The Other Side of the Wind - Orson Welles should be Lord Daemon of Production Hell. After Citizen Kane, he went on to be involved in other troubled works like the Magnificent Ambersons, Touch of Evil and other works. The Other Side of the Wind is his opus from the 1970s. Unfortunately, he made a bad choice in the financing, by getting backing from the brother-in-law to the Shah of Iran on the eve before the Iranian Revolution. Years after his death, Peter Bogdanovich took up the cause of the film. In January 2010, Bogdanovich said he doubted if the work could ever be edited into a usable state.