I started having problems with my teeth. Spontaneous abscess that resulted in multiple root canals. My dentist did some looking into what the cause might be and found some really odd abnormalities with my incisor roots and nerves. When my next appointment came up, he was really quiet for a bit before verbally stumbling about.
It turns out that what was happening with my teeth was a classic sign of inbreeding. I brought it up to my mom and she was like, ‘Oh well, yeah, didn’t you know?’ Of course I didn’t know! Turns out that not very far back in the family tree, several of my relatives decided that it was a good idea to get married to one another and no one bothered to mention it.