I can do them one better: my family has a meta-secret recipe. It didn't used to be secret, but it became secret through no effort or intent of our own.

For a long time, decades at this point, my mom has been known for the brownies she brings to events (and, by extension, that my sister and I bring to events). People would rave about them and demand the secret, which my mom, my sister, and I would all freely give out: it's from a box, Betty Crocker, make sure you get the "Supreme" kind and don't overbake it. That's it.

But. But! A while ago, maybe 3-4 years at this point, my mom noticed that the brownie box had changed. She had a stockpile (being known for the brownies, you see) but tried the new box before exhausting the stockpile. And it was not the same. The new recipe had different amounts of every ingredient and it just did not come out right. At all. She called the company to complain and actually managed to escalate the call far enough to get someone to admit a dark truth: the recipe on their end for the mix hadn't changed, but for various reasons to do with what they wanted to put on the "Nutrition Facts", they had changed the instructions of how to prepare them and the amount of the mix that was in each box.

My mom, not leaving this to chance, did a little bit of kitchen science and verified that what came out of the old box (from the stockpile, following the recipe) was either identical or a very close match to what came out of the new box (but following the old recipe), albeit a little inconvenient as you now needed roughly 1.25 boxes' worth of mix to make one batch of brownies. But this was but a minor hurdle for the truly devoted; and it meant that we now had, wait for it, a bona fide secret family recipe. The easiest one ever, since it still involved just making brownies from a box. (And keeping a jar of "remainder brownie mix" for use in the next batch.)

The epilogue to the story is that although the good recipe had been stable for a really long time, we were evidently not the only ones to notice a problem with the new one, because the box recipe has changed on several occasions since then—each time, as far as we can tell, without changing the blend in the underlying mix (because if we follow our Secret Recipe it still turns out fine). Really, I'm just waiting for them to throw up their hands, give up, and return to the original recipe, which would sort of deprive my family of a good story, but everybody would get better brownies so it's a win overall. :)