Impossible by Design

Review a batch of real defects in a mature codebase and one observation dominates:

The correct mechanism already existed, and using it was optional.

The shared helper was there and the handler held a copy of the formula. The guard existed and the new endpoint did not call it. The dedupe registry existed and the send path skipped it. The accessible component existed and each usage re-implemented it. Nobody lacked knowledge. Nothing enforced the knowledge.

This skill is the conversion: take the convention and make the wrong path unbuildable.

The test

After any fix, ask what prevents the next occurrence. If the honest answer is one of these, you have not finished: