Comment by hnedeotes

This is mostly a self made and kept problem. I also worked in codebases where the current developer(s) didn't know why things were the way they were and would take days to dig into why some part wasn't working - but the issue was, instead of taking the time to write tests and document, even if only when doing these point in time fixes, they just did the ad-hoc thing and solved whatever issue there was at the time without any worry whatsoever.

At some point I was tasked with improving the queries/retrieval times and started converting parts of the code and not only was it possible to write tests that would convey the actual meaning of the code and crystallise it but I started finding bugs in the implementation that no-one realized were even bugs because they no longer knew what it should do when queries grew to certain complexity and the search space was in the hundreds of millions of rows that thinking through them just with napkin calcs wasn't cutting it (specially when the bugs were subtle and the results still allowed the program to work "mostly" correctly).