It has been suggested many times that Adolf Hitler suffered from monorchism - i.e. he had only one testicle. Amusingly, people always seem to remember that it was (supposedly) his left testicle that was missing.
The Wikipedia article is pretty verbose and has numerous issues, and I couldn't spot anything definitive on Snopes, so I'm not quite sure where this one stands. Some suggest that it was a rumour started to boost British morale, others suggest that autopsy notes proved it to be true.
Can anyone provide a definitive, or at least comprehensive, answer?
Foot-note: I really am surprised that nobody has asked this on here yet!