Million-to-one bugs are real, not theoretical. They happen during initialization and restart, not runtime. When they trigger, they cascade - 2,500 errors from one root cause. In high-restart environments, rare becomes routine.

Million-to-one bugs are not only real but high enough to matter, depending on which million. Many years ago I had a rare bug that corrupted timestamps in the logs, with an emperical probability of about one to 3--5 million (IIRC). Turned out that that seemingly benign bug was connected to a critical data corruption issue with real consumer complaints. (I have described this bug in detail in the past, see my past comment for details.)