Mm... Not my parents as such, but I found out a couple of things from my mum's side of the family recently that surprised me:
My uncle (Mum's brother) is not the father of my cousin, who is from a previous marriage.
My grandfather (Mum's father) died a few years ago and told his children on his deathbed that before he married their mother, he had been married to another woman somewhere in the North of England, I think... Anyway, there are children from that marriage so Mum was kind of surprised to find out that she has half-siblings running around somewhere, and I might have cousins or half-cousins or whatever they are that I've never met. Apparently he wanted to track them down but left in a bad way so lost touch with them.
My uncle (Mum's brother) is not the father of my cousin, who is from a previous marriage.
My grandfather (Mum's father) died a few years ago and told his children on his deathbed that before he married their mother, he had been married to another woman somewhere in the North of England, I think... Anyway, there are children from that marriage so Mum was kind of surprised to find out that she has half-siblings running around somewhere, and I might have cousins or half-cousins or whatever they are that I've never met. Apparently he wanted to track them down but left in a bad way so lost touch with them.