Higher up the ladder is better.

The Steps

Step 0 - The bug exists, you never learn about it

Users are annoyed but work around it

Step 1 - A user tells you about the bug

Step 2 - Automated logging/alarms tell you about the bug

Step 3 - A deploy is prevented by a liveness/readiness probe catching the bug

Step 4 - During automated integration tests

Step 5 - During automated unit tests

Step 6 - Whilst coding because of the type system

Step 7 - The bug can not exist in your code because of the design