Watching the opening cermonies at the Athens Olympics, I just knew someone would mention the "legend" which underlies the marathon - the Athenian army beat the Persians, and this one dude ran back to Athens to share the good news, and promptly dropped dead of a heart attack - and then hint that it's probably just a cool story. When I was reading up on the Greco-Persian Wars a couple years back, I discovered that the truth is actually more interesting: The "marathon myth" is wrong for a different reason. According to "The Greco-Persian Wars" by Peter Green (an eminent classicist), what happened was this (I'm summarizing; this is from pp. 35-40):
After the Athenian forces defeated the Persian land force, the rest of the Persian fleet sailed around with the intent of attacking Athens. The entire Athenian army then hightailed it back to Athens, beating the Persian fleet by an hour or so. The sight of this made the Persians retreat, and made pro-Persian Athenians unwilling to betray their city. The Pheippides story does indeed seem to be myth, but there's an apparently true story which is actually even better. BTW, they ran 24 miles, not 26 - the discrepancy has to do with whether you measure from the modern-day town of Marathona versus the ancient battle site. Still pretty cool though.

Why does the false legend have such persistence when the truth is so much better?