I found a link via the Skeptical Inquirer (May 2000) to a bunch of humorous mass delusions throughout history. For instance:
Mackay (1852, 539-540) reports that in 1639 at an all-girls’ school in Lille, France, fifty pupils were convinced by their overzealous teacher that they were under Satanic influence. Antoinette Bourgignon had the children believing that “little black angels” were flying about their heads, and that the Devil’s imps were everywhere. Soon, each of the students confessed to witchcraft, flying on broomsticks and even eating baby flesh. The students came close to being burned at the stake but were spared when blame shifted to the headmistress, who escaped at the last minute. The episode occurred near the end of the Continental European witch mania of 1400 to 1650, when at least 200,000 people were executed following allegations of witchcraft.
If you’ve never picked up Charles Mackay’s book, Extraordinary Popular Delusions & the Madness of Crowds, it’s quite the entertaining read. Mackay tended to believe that when people got together, their intelligence dropped significantly, and his book provides lots of anecdotes to prove his case.