Austrian-born Erich von Stroheim's 1924 silent film Greed (based on Frank Norris' 1899 novel McTeague) is regarded by many to be one of the greatest films ever made, despite the fact that more than half of the movie was cut before its mass public release. Depending on who you ask, the original cut ran for 8 or 9 hours (some quote an even longer run time) and ran over 42 film reels, but was cut down to about 4 hours (24 reels) by Von Stroheim himself (partly against his will), after which time he insisted that he could not (or would not) cut another single frame of footage from the film.
The film was then given to Von Stroheim's colleagues film director Rex Ingram and editor Grant Whytock, who cut it down to an even shorter 140 minutes (18 reels), before Ingram sent it back to Von Stroheim, who stated "if you cut one more frame I shall never speak to you again". The 140 minute cut was screened to the public, and became the definitive cut of the film, although in 1999, a 4-hour cut was released, with the aim of being closer to the director's original vision, although since by that time the additional scenes had been lost, still photographs were used in their place.
Only 12 people have seen the full 8 hour cut at a special screening in January 1924, many of them proclaiming that they had just witnessed the greatest film of all time and that it was unlikely a better film would ever be created. It's not entirely clear how the additional scenes became lost (allegedly, the footage was destroyed by a janitor), although many rumors of existing copies have been spread and some later disproved. Von Stroheim himself has confirmed that Benito Mussolini was once in possession of a copy of the original cut, but the fate of this copy (as well as the other rumored copies) is entirely unknown, and none of the lost scenes have been publicly discovered to this day. The original 8 hour cut has been dubbed by some as the "holy grail" of film archivists.