look up 23&me and Ancestry.com horror stories.And hold on...
MANY?!?!
ALOT of people have found out dad and bio-dad are two different people. From what recall reading, all of the stories involved families where mom and dad were married.
To be fair, folks also found out they had half-siblings they didn't know they had cause Dad was a secret playboy.
It should be noted a lot of the stories I read seemed to involve full grown adults (40+ age range). Many had lost the parent they would need to talk to for answers. In some cases, it's possible both parents knew the deal but kept quiet for whatever reason thinking it would never come out. Ofc, alot of stories came down to Mom/Dad were secret ho's.