But I think it should be posted under "the shady side of history - the royals".
People who claimed to have been a missing/ murdered/ died monarchs or his/hers accendants have been not so uncommon as one might think.
Besides the ones mentioned in the previous post (Louis XVII, children of George III) there have been plenty of others.

Long after his death, there have been people who claimed to be
Tsar Peter III (husband of Catherina the Great)
And the same think happened in Bavaria, where long after King
Louis II (Ludwig) drowned in a lake, people claimed to be him.
They said they escaped a "brutal assasination" and waited for safer times.

I'm sure there are more stories like that. I think it would make a great topic!