This picture shows the call structure of a medium-sized (!) Cobol software system. The red dots each represent a program, the lines are the dependencies between the programs.
The technical debts can be easily seen here because of the large number of dependencies. The difficulty in practice for a developer is that he has a program and possibly also the adjacent programs in his field of vision, but it is impossible to have an overview of the entire system.
What do technical debts do?
Errors and unwanted side effects of changes in one program can affect the entire system. As a result, the time required to analyze possible effects often reaches incredible proportions. Strongly person-dependent know-how Know-how about the internal relationships in such systems is strongly person-dependent. Bringing employees back from vacation or reactivation from retirement are (necessarily) possible consequences.