My dad learned as an adult that his father, who he’d known as is dad his entire life, was not his biological father. But that’s not the crazy part.
After my grandmother died, my dad asked me to see if I could find anything out about his biological father online. Popped a few details into ancestry and very quickly found a marriage certificate showing a marriage that happened before my grandfather. Then I found another. Then ANOTHER.
My grandmother had been married not once before, but THREE TIMES before she eventually married my grandfather. And she’d kept them all secret. We don’t even think my grandfather knew about the first two. This was all in their 40s, spanning several states, and we weren’t able to find divorce paperwork for all of them, so it’s quite possible she was legally married to several people!