[mahemm]

Sounds like insufficient testing to me. This kind of deep/far reaching issue should have been crashing tests written before others would have seen it.

Assuming that there is some testing in their work (if not; there's the answer), my guess would be that it's largely sanity testing of each functionality in a vaccuum. The coaching would probably be to demonstrate more integration-style testing, possibly using whatever tests caught system-level issues that the dev has pushed in the past.

[potta_coffee]

Could also be organizational. Constantly shifting whims of stakeholders, etc. Yes, requirements change but changing major requirements without a major refactor can cause these kinds of problems with the code. Or if the developer is being given unreasonable deadlines, no time/resources for testing, etc etc.

[EpicEng]

>the problem space misunderstood and everything patched over with little fixes to get things working for a particular case.

Sounds like testing found issues which were hacked over, but more will keep popping up because the design is inherently incorrect.

[kyberias]

No, it's just lack of true skill.