When I was around 15 I learned that my moms father was actually her step father. He had already passed but when alive he was the only grandfater I had so the big secret had no impact on me.
Here's one I just learned about a couple of months ago. My dad was forty when he married my mom who was seventeen, and pregnant. What I just learned is he never told my mom he was married before and had five other children, that didn't become known for many years and when my mom found out she threw him out. This was before I was born but one of my older brothers told me they lived apart for a couple of years before she took him back.
Growing up I knew he had another family but it was rarely discussed, now I know why.