The interesting kind of technical debt is when you realize you modeled the problem wrong, or the problem has changed and you need to change the model. A simple example is an auth system that assumes a one-to-one correspondence between account and user login. Then you get, for examples, agencies that are conceptually one account but have multiple logins (so each client can only see their own account). Oops, that breaks the model. Changing this requires database migration, code changes, UX changes, and so on. It's a conceptually simple change but has wide implications.