I have a cousin who found out, in his 50s, after his mother and father had passed away, that he was not related to anybody in our family. It crushed him.
My family found an estranged relative from a one off affair in another country through Ancestry's "probably related" tool. It was an incredible experience.
Sample mixups and other mistakes can happen. I know of cases where family history could become exposed through these tests, but if you get results that are unexpected it's worth some follow up to be sure.
I have a cousin who found out, in his 50s, after his mother and father had passed away, that he was not related to anybody in our family. It crushed him.