It's a conceptual error with the problem, which means that it indeed WAS missed by unit tests. I wrote some of those and checked the "overlapping numbers" case; the difference was that I did so in a naive situation where the values were inserted once and never updated.

Now, it would have gone from a "mysterious" error to an obvious one had I done an academic-style step-by-step visualization applet of the entire structure's processes before integration, but I had confidence when the code was first written that because this structure was being used with extremely high frequency, the only thing I had to write an explicit test for off the bat was off-by-one ranges causing edge-case "near misses"; anything else would make itself known at runtime after integration.

Time it would have taken for a visualization applet: ~3-6 hours? Time it took to solve the bug: ~3 hours

It's placing a bet - and the bet is that the bugs are limited to a specific segment of code that can be narrowed down easily; if the problem were architectural in nature I'd be in far deeper shit because that would make the same class of bug appear all over, with any number of different symptoms.