I was trying hard not to make a strong assertion, so I'm not sure what you are disagreeing with entirely... TL;DR - it's a matter of context But here's an ane

I was trying hard not to make a strong assertion, so I'm not sure what you are disagreeing with entirely... TL;DR - it's a matter of context But here's an anecdote where two complex pieces of code interacted... A few years ago I was working on a fairly big project (~50 developers) and was responsible for much of the architecture, and on an implementation level, implementing some of the lines that connected boxes in said architecture. One of those was a line that crossed what can loosely be thought of as a system/application boundary, this involved both implementing the application hosting layer and writing the marshalling code that moved data in either direction, there was a bunch of complexity because of not quite convergent type systems, some threading issues etc... So all this is done, not too many tests have been written, but hey, it works, if it didn't nothing in the system would work... a year or so into the project, the most senior technical person on the team has a flaky test that once in a while just completely fails to make progress, he investigates, and comes to the conclusion that once in a blue moon, the transport layer (my code) is dropping a message. Needless to say I'm totally flummoxed, there is no code path that can drop messages without also creating a huge stink in logs etc. I spend two months investigating this, eventually I have a consistent repo, a fairly big hole in the other guys state machine that causes it to just stop making forward progress. So despite the fact that a test exposed the bug, neither piece of complex code really had proper tests, but I would assert that the "line" didn't need the same level of tests, it was being exercised constantly by dozens of "boxes", but the unicorn "box" with the logic error could sure as hell have used some unit tests to validate its state transitions.