In 1184 a document called the Toledo Letter appeared and rapidly spread throughout Europe. It predicted that an ominous conjunction of the planets signalled the imminent end of the world in September 1186.
When the letter reached the Archbishop of Canterbury in England it prompted him to order a 3-day fast.
September came and went and the world didn't end, but the letter continued to be circulated for another 100 years with the doomsday date changed.