One of the most dramatic (and dumbest) conspiracy theories of all time is the Phantom Time Hypothesis, put forward by the conspiracy theorist Heriber Illig. They hypothesis states that almost three centurires of the Middle Ages, AD 614 to 911, never happened, and it was all because of Otto III (pictured below) and Pope Sylvester II.

Like the legend of Polybius, though, this is a conspiracy theory with some fascinating truth behind it. There are indeed chunks of missing time in the calendar, a result of the switch from the Julian to the Gregorian systems.