On an embedded system, we had this bug that we couldn't find. It was around for a month or two. Random crashes that we couldn't reproduce, couldn't even debug. We started calling it "the phantom".
Finally Ed said, "I think the phantom showed up after we made that change to the ethernet driver." We reverted it, and the bug disappeared.
We never found the bug in the source code. But Ed debugged it using the calendar.
Pulled my hair out for a year, no progress/insights. Updated the driver for a device, haven't seen it since.
I hope the reverse-calendar debugging works for me!