In my attempt to be a conscientious tester of a new feature in our codebase, I put in some stub code to allow me to test locally and hit all the various boundary cases.  In a painfully ironic twist, that test code introduced a bug which only reared its head in a small set of boundary cases.  Note that all of our operational tests after deployment test transactions with the main path.

So, in being a good(?) tester, I broke the system.  The fun of pushing bits.