The claim that Origen castrated himself involves his supposed literal interpretation of Matthew 19:12.
Dammit, you beat me to it. That's one of the best stories.
For there are eunuchs who have been so from birth, and there are eunuchs who have been made eunuchs by others, and there are eunuchs who have made themselves eunuchs for the sake of the kingdom of heaven. Let anyone accept this who can.”
I know the quote is really about celibacy, but it's very easy to understand how one could have taken it literally.