This feels like a good moment to mention my most embarrassing bug which was, of course, in a billing system. I had written code to handle the overdue billing of customers' usage on an internet telephony service. The code seemed to work great in QA so we went ahead and pushed it to production.
A couple days later we got an angry call from a customer whose card we'd maxed out. The final stage of the process was to adjust their bill by the amount that they paid, but there was a sign error in that adjustment so instead of lowering their balance by the amount paid, we increased it. As a consequence, for a user with, say, a $50 balance, we kept doubling the amount that we charged them each day.
A couple days later we got an angry call from a customer whose card we'd maxed out. The final stage of the process was to adjust their bill by the amount that they paid, but there was a sign error in that adjustment so instead of lowering their balance by the amount paid, we increased it. As a consequence, for a user with, say, a $50 balance, we kept doubling the amount that we charged them each day.
Exponential growth is a powerful thing.