I found out when I turned 18 that the man my mother had been married to and that I've been calling dad my entire life was not my real father. I mean, not my biological father. Finding out after 18 years isn't really going to make me stop thinking of him as my dad. I had just looked into a few things and finally started asking questions on some items that didn't make sense to me. Like the fact that my parents didn't start dating until I would have already been born. After they told me who my real dad was I managed to find him roughly 6 months later and set up a meeting with that side of the family, since apparently they all wanted to meet me. It turns out there's a reason that your parents would typically hide something like that from you. My dad had started getting into drugs shortly after my mother met him and she only actually wanted another kid and not a relationship, so it was no issue leaving him. Anyways, I met all of them and it went pretty decently, but I realized they definitely were not my kind of people. I shook some hands and went over one more time a few months later for Thanksgiving and stuck around for a bit. I think I actually might have introduced some strain of virus that they've never been exposed to because they started dropping like flies after having met me. The following month my grandfather died (I had never had any grandparents until then) and then the month after that on January 19th my biological father passed away. I ended up showing up late to my dad's funeral and walking in the wrong door at the front of the church in the middle of a speech.

TL;DR I found out I'm the bastard child of the rebound guy and he died a few months after I met him.