Logic Errors

A newly built system's rules often are few and optimistic in nature, and so spotting mistakes within them is easy enough.

But those rules inevitably evolve as a system grows. And because their edge cases tend to multiply, subtle flaws accumulate that are only obvious to those with enough domain knowledge to spot them.

These errors often don't cause systems to crash nor do they necessarily cause old tests (which are based on simpler use cases) to fail, and that makes them especially challenging to find and fix.