Sometimes there were reasons, but they were not necessarily good ones. Like these: - time pressure, leading to just making it work and not caring about edge ca

Sometimes there were reasons, but they were not necessarily good ones. Like these: - time pressure, leading to just making it work and not caring about edge cases, which were bolted on later. Or maybe never at all. And naturally, there is no useful testsuite. - the prototype ended up in production - a refactoring happened, but there was no follow-up into other areas of the code base - features changed, but the code is still influenced by the necessities of the old way it worked. Similarly, for changing technical circumstances - code was copy-pasted or was worked on by too many people - code was herded too long by the same person and now, to put it nicely, bears traces of their bespoke coding style Chesterton's Fence is still very relevant, therefore I'd still err on the side of writing a lot of high-level tests before touching anything.