How many of your checks have ever returned False?

The author shares their experience developing a system for analyzing manufacturing instructions equipped with a dangerous phrasing detector. During operation, it became clear that many quality checks were malfunctioning or failing to detect violations entirely. To diagnose the issue, a special tool was created to intentionally introduce errors into documents. The results were unexpected: half of the checks failed to detect any issues. Further investigation revealed that the diagnostic tool itself contained errors, including configuration issues and incorrect result logging. The author emphasizes the importance of regularly testing quality control tools, as even verified code can hide 'blind spots' that render a system ineffective. This case serves as a reminder to developers about the necessity of rigorous logic verification and regular audits of test coverage to ensure that the system is truly performing its intended functions.