I was quite certain there were no unknowns in my family, but my DNA kit accidentally revealed my great-grandfather had a child with another woman while his wife was pregnant with my grandma's sister. I had loads of second cousin matches coming up, and they all had a missing grandfather in their family tress, because the mother of the children in that family didn't put any father's name on the birth certificates. From what I know now from my new cousins, she had a bit of a troubled life, and quite a sad story.

My grandma moved away quite young so we don't know much about her parents, so it's hard to say whether his infidelity was known and pushed under the rug, or if he was openly pursuing other women. Either way, I doubt my grandma ever looked back, since none of her children (my aunts and uncles and dad) have ever known much about her life before she was married.