I once helped one of my company's customers troubleshoot an issue that had seen the same ridiculous edge case error happen three times over the course of a few years. At one point the actual sustaining developer we worked with was able to narrow down a specific bit that was getting flipped somehow, and pitched that cosmic radiation was a plausible solution given how rarely this kind of thing impacted other customers.

It was at this point that we remembered that the customer was either a university with a nuclear physics lab or a hospital with a nuclear medicine program (can't remember now, ironically enough) that the server rack lived adjacent to.