It was a long time ago, but I remember we were working on an embedded system controlling some industrial equipment, and it randomly crashed; the time between crashes was long enough that it'd take several days before it happened, so even getting a trace of the crash was an exercise in patience. Eventually we did get a trace, and it turned out the CPU would suddenly start fetching instructions and executing from a completely unexpected address, despite no interrupts or other things that might cause it. We collected several more traces (took around a month, because of its rarity) and the addresses at which it occurred, and the address it jumped to, were different every time. Replacing the CPU with a new one didn't fix it, and looking at the bus signals with an oscilloscope showed nothing unusual - everything was within spec. We asked the manufacturer and they were just as mystified and said it couldn't happen, so we resorted to implementing a workaround that involved resetting the system daily. Around a year after that, the CPU manufacturer released a new revision, and one of the errata it fixed was something like "certain sequences of instructions may rarely result in sudden arbitrary control transfer" - so we replaced the CPU with the new revision, and the problem disappeared. We never did find out what exactly was wrong with the first revision, other than the fact that it was silicon bug.