Sometimes the logging is not enough, and you cannot isolate the case to prepare appropriate unit test.

Just few days ago I had a strange problem with the order of imports in Python at the border of my code and external library (Celery). There were import hooks involved but they didn't seem to be executed properly in certain conditions. I could reproduce them quite reliably but I needed to pinpoint the exact import (inside Celery itself, mind you) that was causing the problem. pdb (Python debugger) was indispensable while solving it.

On the other hand, though, it was probably the first time in many months that I used pdb for more than 5 minutes, and for something more complicated than checking why a particular test fails.