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You have some lovely stories online about these things. Like the guy tracking down a stuck bit in RAM.

Or on the network level, a VPN that failed only when traversing one possible route between company offices.

I have a very old one. I was working on a embedded system using an Intel 80186. The ‘186 had a memory mapped IO system that was very unusual for Intel x86 CPUs. The code that I started with was originally written by a bunch of Motorola 68000 guys so they used this mode. I had to modify the interrupt structure so I rewrote the interrupt service routines. Since it was memory mapped and ANDs were documented to be atomic, I assumed that I could just AND bits into the interrupt mask register. Big mistake. It turned out that ANDs to the IO registers were not atomic. And could be interrupted in the middle of the write after read. I took me about a month to realize that this was a hardware “bug” and not in my interrupt code. Drove me nuts.

These problems have existed for years - I once tracked down a dead cell in core memory on a system I worked on in the Air Force. Because of the chance alignment, it caused incoming messages to never "end". Which was fun for the operators.

http://www.1882nd.com/images/DSTE.jpg

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