The end is nigh, says Christian radio broadcaster Harold Camping, who has spent the last year blanketing U.S. cities with billboards and leaflets claiming the end of the world will begin on May 21, 2011, as 200 million people are “raptured” up to heaven. When his followers wake up on May 22, the rationalizations and recalculations will begin.
James Randi's Encyclopedia of Claims, Frauds, and Hoaxes of the Occult and Supernatural tells of 44 similar doomsday predictions that, predictably, flopped. In many cases, the false prophets didn't go bankrupt after flunking these all-or-nothing predictions. Instead, their followers became even more devoted.
Here's one entry from Randi's list. You can read the rest here.
April 3, 1843 (And also July 7, 1843, March 21 and October 22, 1844): William Miller, founder of the Millerite church, spent fifteen years in careful study of the scriptures and determined that the world would conclude sometime in 1843. He announced this discovery of what he called "the midnight cry" in 1831. When there was a spectacular meteor shower in 1833, it seemed to his followers that his prediction was close to being fulfilled, and they celebrated their imminent demise. Then, as each date he named failed to produce Armageddon, Miller moved it up a bit. The faithful continued to gather by the thousands on hilltops all over America each time one of the new dates would dawn. Finally, on October 22, 1844, the last day that Miller had calculated for The End, the Millerites relaxed their vigils. Five years later, Miller died, still revered and not at all concerned at his failed prophecies. The movement eventually changed its name and broke up into a number of modern-day churches, among them the Seventh-Day Adventist Church, which today has over three million members.