This afternoon, I found a duplicate profile in my tree on Ancestry.com. "Oh," I thought, "I'll just merge these two since the details are identical."
...and that's how I discovered that my dad's parents were born the same year, to the same mother, but two different fathers. Um... ooopsie? She did at least marry them both, fortunately not at the same time.
I can laugh at this because not only is there no one left on this branch of the tree but my brother and me, neither of us has children nor intends to. Nevertheless, it did come as a bit of a shock!
While I don't expect most people to admit it if their father was the result of incest, I'd love to hear what sort of unexpected surprises others have found in their family trees!
(I'm still waiting for my GEDCOM to be approved, but I've entered the relevant bits manually already.)