I'm not sure if this qualifies, but I found it interesting at the time I solved it (Spring 2011?): https://cameronwatters.com/writing/2017/08/01/take-the-time-...

TL;DR (of the problem): customers from a single country—and only that country—were (sometimes) unable to checkout on an e-commerce website and, after months of trying to sort out the issue, the solution turned out to be very facepalm-worthy.