...what was/were the most surprising discovery(ies) you've made when investigating your family trees?

For me, there was a notable recent one (both from a generational standpoint and regarding when I found out about it, only last December). When certain books of birth records and indices of birth, death and marriage records were made publicly available online, I found out that my maternal grandfather had been named after an older brother of his (given names only; my great-grandparents went on to give different surnames from those of their late firstborn to my grandfather and later my great-aunt), who was born the year before him and only lived for a day.

Now, being named after a deceased older sibling was a very common situation in the past, but what was surprising about this was that my grandfather never once mentioned this brother (and not only in my recollections; my 76-year-old uncle was shocked to learn about this uncle of his). In contrast, my maternal grandmother would often mention her older siblings who died young when talking about her family. Since, as my uncle said, my grandfather was not the kind of person who'd keep this information secret from the rest of the family (and he was never interested in genealogy), it's very likely he himself never knew about this brother of his, and so the latter's existence basically remained a secret for 107 years!