I found out my father was an inventor with quite a few patents to his name. I don’t think he had ever told me that!
Some interesting stories.
I have no definitive proof of this, but I think I have an older 2nd sibling. My birth certificate says that my mother had two prior deliveries and my older sister’s birth certificate says that my mother had one prior deliveries.
I do know that Mom had a early term miscarriage before my sister was born but I don’t think that counts as a delivery.
I only discovered after I got a notarized Birth certificate for my passport, well after my mother died. My still living father denies this and assured me that my mother was “very virtuous”.
My father is my last living relative of that generation so there is no one else I can ask.
I only learned the correct spelling of my mother’s name about 5 years ago. She always spelled her first name as two words and it wasn’t until I saw her wedding license that I noticed the different spelling and she told me that was the correct spelling. While it’s not exactly as dramatic as those above, it’s sort of a shock to realize you didn’t know your own mother’s name.
Not my parents but mischievous grandpa, resulting in an uncle whose identity is lost to history.
Then there were my slave owning ancestors. We have a lot of cousins.
Yeah. Lots of folks wonder about Dad, but Mom is usually pretty straightforward. 
Not me but …
I have a friend who found out after his Dad had retired that he had worked for the CIA. As Q (*a la *007). During the exciting era of Cold War shenanigans.
Before that news, Dad had always seemed a quiet nerdly engineering sort. But he’d built some pretty diabolical thing-a-ma-jigs.
It’s never been a secret that my brother and I were both born exactly nine months after New Year’s (different years). But it wasn’t until shortly before she passed away that my mother told me the whole story. The fact is, my father needed alcohol to get him into the mood. The problem was that he rarely drank. So my mother would wait till New Year’s Eve to get a few drinks into him. She wanted to have three babies, but it worked only twice.
That’s sad. 
Heck, I have to get a few drinks in my dog just to get him to sit on my lap.
Dad was the second man who proposed to Mom.
Also, one of my grandfathers was married three times.
My father had secret children that he supported financially, but never contacted, or even mentioned. This was especially shocking since I could call him right now and ask for nearly anything, and he’d do everything in his power to give it to me.
Found out at my dads funeral that he’d sent a generous monthly check to his mother for decades until she died. Not a surprise at all. Just never knew it.
It wasn’t until I was 13 and had discovered psychedelic rock on my own that I found out my father - my socks and sandals and dorky ball cap wearing, jazz and folk listening, Star Trek watching, king nerd father - had been at Woodstock. I was talking about Hendrix and this really awesome version of The Star Spangled Banner he played at Woodstock, and Dad - who, at that point, was a project manager at a defense contractor - just said, “Oh, yeah, that was the greatest part. We were really glad we stuck around for him.”
Honestly, it pretty solidly changed my perception of him. He burned me a CD that I still have; his favorite tracks from the festival interspersed with him narrating his memories of it.
As both parties are dead now (Mom just died a few days before Mother’s Day), I can reveal a few tidbits I didn’t learn until years later:
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When my parents separated, it was partially because my dad was cheating on Mom with the woman who would later become my stepmother. When he eventually moved in with and became engaged to Jan (stepmother), he cheated on her with my mother.
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A few years before he died, Dad and I were talking about things in general. He asked if I’d ever tried anal sex (I, to this day, do not recall how this topic came to be in the conversation). I asked him if he meant giving or receiving. He looked at me weird (I’m straight, but I have a tendency to make jokes that sometimes leave people wondering. Not that there’s anything wrong with it), and said giving. I said no, as - at the time - I never had. He told me that it was something I needed to do at least once. He then revealed that on their first date, he and my mother engaged in anal sex, which put all kinds of unpleasant visuals in my mind.
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Dad was in Vietnam, drafted into the army, where he was in artillery. He never talked out his experiences. It wasn’t until he died that I learned that he had mortared a passenger bus that their intelligence said was filled with troops in the Northern Vietnamese Army. It wasn’t. It was actually a bus of children and women (at least, that’s the way it was relayed to me. It’s possible that the actual truth of this incident has been distorted by memories and such).
ETA a story about my great-grandmother. She married into a family that had a very Dutch last name. When she divorced her husband, she didn’t want him to be able to find her and their son, so rather than keep his name, or go back to her maiden name, she changed the spelling of it. When she did, it changed the name from Dutch to German. I have a very German last name as a result, but no German blood.
Found out that when my mother was a small child, she was abducted and taken out of the country by her mother. It took over two years for her mother and father to reconcile (and it was apparently somewhat a “forced” reconciliation) and for her to be brought back to this coumtry.
ETA - my mom is really cool once you get her talkin! 
I didn’t find out until I was out of college that my dad had also dropped out of high school to join the military. He fought in the Pacific Theatre. He was selected for OCS but couldn’t enroll because he didn’t have his actual diploma and they wouldn’t accept a GED.
I also discovered that he’d been married for a brief time before meeting my mother. Trouble was, even though they’d been separated for a while, neither of them bothered filing for divorce. In order to marry my mother, he had to come clean. His ex never contested it. He never knew what became of her. When I asked my mother if he and the ex ever had children, my mother said she had no idea because my dad never mentioned anything. They probably didn’t, but there’s part of me that’s always wondered if I’ve got a half sibling out there.
My parents married in 1946. For a long while I had the impression that their relationship had come to be, slowly and gradually: they, and their respective families, had known each other well and seen plenty of each other, since childhood. Have discovered recently from various family correspondence which has been kept since that time, that their deciding to marry was sudden, and a great surprise to all who knew them.
They had presumably seen each other now and again during World War II: my father was in the British merchant navy, my mother in the UK throughout. Their marrying, though, shortly after the war, was – as said – highly unexpected. The marriage was happy enough, so far as is known: it ran only for some twelve years, with my father dying suddenly and unexpectedly in his early forties. There appears, though, to have been some feeling among their contemporaries, that they seemed not obviously very well suited as spouses for each other.
There is some opining in the correspondence mentioned, that their marrying was a snap decision, contributed-to by euphoria at the war’s being over. Less-pretty scenarios can be speculated on – if an unplanned pregnancy had been involved, it must not have come to term: I, the eldest of three siblings, showed up only in summer 1948.
I found this out as a teenager:
Before my mother met my father, she dated around a little bit (and, really, that’s all it was. Her mother would force her to keep a can of food in her purse - in case the guy got “fresh.” Mom also had to take at least one of her siblings with her on every date she went on, in an attempt to keep her “pure.”). She used to spend a lot of time at her aunt’s house, and met a boy from the neighborhood. They had a few innocent dates (from what I was told), and she really liked him. But he just didn’t feel the connection, so they went their own separate ways.
A few years later, she sees where he got married to another girl from the neighborhood. My mom sends him a card congratulating them.
Fast forward a few more years, and she sees where he was just convicted of killing his wife, and sentenced to life in prison with no chance of parole. In an attempt to avoid prison, and to try an insanity defense, he took a pencil, and stabbed himself in the eye. It didn’t work. And he lost the eye.
My parents adopted me from the circus…at least that’s what my older brother has always told me.
I really, really hope my kid never sees fit to post in this thread. Especially any time soon. 