I just fixed a bug in one of the modules powering this web site.
It started with a very simple symptom. When creating a new issue in the bug tracker, I got the following PHP E_ALL notice:
Undefined property: stdClass::$cid
Unfortunately, fixing the bug was not a simple matter of suppressing the warning. At first, I couldn't even reliably reproduce the symptom. I had to investigate more in depth and understand the exact circumstances that lead to the faulty manifestation. I finally found the proper way to fix that particular bug.