I'm not sure how this classifies as tech debt, but in my experience the worst type of tech debt is found in parts of a system that is relied upon and whose busi

I'm not sure how this classifies as tech debt, but in my experience the worst type of tech debt is found in parts of a system that is relied upon and whose business logic has become unmaintainable, is too complex/poorly understood and is not possible to throw away as too many things depend on it. Also usually whose initial creator is long gone, the documentation is conflicting with the code implementation. This kind of debt causes eternal confusion and constantly lowers the productivity of the team. In cases like these, workarounds are created and more tech debt accrues on top of it that is even harder to reason about. Nobody sane can fix a mess like this and the only hope is death of the system or complete overhaul.