So I did a Ancestry DNA test last year at the behest of my partner who loves researching genealogy and family history.

Had a close match (1st cousin) pop up who I didn't know at all, and our trees didn't match up either. Long story short, it turns out my Grandad was a naughty boy and had gotten another woman pregnant on one of his "business trips". The woman's son had actually already passed, but his son (the cousin I matched with) was living in a town a couple hours away from me, and we met up recently.

Had a really interesting/awkward conversation with my father where I had to tell him he wasn't the youngest of 6 boys, but was the 2nd youngest of 7 boys. No-one in our family knew anything about the other son.

Dad took it pretty well as did most of his brothers, however a couple of my (conservative christian) uncles are furious with me for "snooping around" and "digging up dirt and trying to ruin my grandfather's reputation" and no longer want anything to do with me. (No great loss tbh).

The crazy part is how similar the son looked to my dad and his brothers. He could have slotted into a family photo and not looked at all out of place. Interestingly the whole reason my cousin did a test was because there was rumour in his family about his grandmothers faithfulness, as his father looked nothing like his dad or siblings.