In my early Linux system administration days, when I was first trying to set up a mail server with spam filtering, I ran across a really puzzling bug in Dspam, the software I was trying to get working. While all the other users of the software were getting great results, with Dspam catching 99%+ of all their spam, it was only catching about 70% of my spam after quite a bit of training.
I posted my results, and confusion, to the Dspam mailing list. The original developer of this software (which has thousands of users), Jonathan Zdziarski, responded that that did not sound right. He asked if he could log into my server and see what was wrong.
I created a test account for him, and logged in to the same screen so I could watch what he was doing. As I watched, he put debugging messages into his code, ran several tests, and within 10 minutes had identified the problem: the 19-digit number he put into the database (MySQL) was different than the 19-digit number he got out. It was a storage error in somebody else's software causing the problem, a rounding error. He filed a bug in MySQL, and it was fixed within a month or two. But he also had a workaround for me: change the data type to a set of characters instead of an integer. Problem solved.
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