My grandmother always told us that her grandfather was named "Jake Biggins" (names and other details anonymized to protect the guilty) and that he came from the state of "Podunkia". There was extant documentation in a family Bible that her father was "Jacob L. Biggins" and that he had in fact legally married her mother in "Anytown, Homestate".
Tracking "Jacob L. Biggins" through the US Censuses, I found him in combination with his mother "Lucy" and an older sister "Geraldine" and younger brother brother "Fred", both in "Homestate", and earlier in "Notown, Podunkia".
Family tradition (i.e. my grandmother) had it that "Jake Biggins" had served in the Civil War, and I was able to identify a man by that name who had enlisted from that part of "Podunkia". The difficulty was that, when I traced his further details, he had gone off after the war to "Otherstate" and married and raised a family there.
Bigamist? Or mistaken identity?
While I'd been tracking "Jake Biggins", I'd found out that he originated from the "Northeast Corner" of "Podunkia", and I had found reasonably solid documentation on his ancestors for two generations back.
Following his descendants in "Otherstate", I found that his son "Algernon" had married a "Geraldine Biggins Biggins" from "Notown, Podunkia", and that her mother's name was "Lucy Wilkes". Errr....what?
Back to the Censuses. Sure enough in the later Censuses, there was "Algernon Biggins" in "Anytown, Homestate", married to "Geraldine" and with mother-in-law "Lucy Biggins" living with them. ("Jacob L. Biggins" and his wife - and their family, including my grandmother - were living nearby, in the same town, so that was almost certainly Great-Great-Aunt "Geraldine".)
Back to the "Northeast Corner" of "Podunkia" and whatever vital records might be available. Found a record that in 1860 a "Lucy Wilkes" had gotten married...to an "Ezekial Biggens". A bit of backtracking turned up an "Ezekiel Biggins" and his older brother "Zebulon", who had been raised by relatives-in-law after their parents died and left them without a bean. Neither "Ezekiel" nor "Zebulon" had Civil War records, as far as I could find, but "Zebulon" had had a wife and son on the "Northeast Corner" and they had left descendants. "Ezekiel" had, to all intents and purposes, just disappeared.
Ooooo-kay. So apparently Great-Great-Granddad's name was "Zeke", not "Jake", and the "Jake Biggins" who had served in the Civil War was apparently a great-great-great-uncle. And if I hadn't cross-checked extensively, I might have gone on barking up the wrong family tree and getting more and more confused until I just gave up.
Persistence pays. So does diligence. And "family history" should be trusted only as far as it can be validated.