This is a working example of using red/green testing to fix a bug - one that starts in the issue description and lives in the code. You can fix the issue, but the bug remains because it's part of an unexamined specification. Testing - and caring - are the fix.
Most teams don't trust their code - they trust the people who wrote it, or the streak of days without incident. That's not the same thing, and the difference is becoming harder to ignore. Trust in a system isn't social capital or accumulated momentum: it's demonstrated, repeatedly and verifiably, by the system itself. If you have to read the code to know it works, you don't know it works.