I've also traced my ancestry back to Charlemagne. It's good fun but it's not the iffy documentation that is the main problem. It's the milkman effect. In talking with a researcher at a genetic genealogy center he said that around 10% of the population has a different biological father than they think they do.
I'd love to see this either substantiated or debunked - I've seen this claim made and refuted many times, but have never found a truly reliable source.
My recollection is that this number is based on a percentage of individuals seeking paternity tests-- where obviously we would expect to see a much larger number of false paternities showing up than in the general population.
