Great story! Yup, this kind of thing happens. For some reason it reminded me of something that happened to me as a newbie engineer. It was really funny a week later.
I was troubleshooting an intermittent problem in a piece of equipment. It had several boards full of mostly LS TTL logic chips (yes, them chips). It was the kind of problem that only happened once every other day or two. Nobody knew. So, I had all kinds of instruments attached to this thing and was watching it like a hawk waiting for a failure. It had probes attached to every point in the circuit where I suspected I could see something and learn about the source of the problem. I also tested for thermal issues with heat guns and freeze sprays, familiar troubleshooting techniques to anyone who's done this kind of thing.
Anyhow, every so often the thing would go nuts. The three scopes I had connected to it showed things I simply didn't understand. I'd analyze but couldn't make any sense out of it. Still, again, every so many days it would happen again. Changed power supplies and the usual suspects. No difference.
Well, finally, two weeks later, the other engineers in the office took pity on me and told me what was going on: They had connected a VARIAC to the power strip I was using to power the UUT (unit under test). The scopes and other test instruments remained on clean power. Every so often they'd reach into this drawer where the VARIAC was hidden and lower my power strip's voltage just enough for the power supplies to fall out of regulation and everything start beeping and sputtering. Those friggin SOB's. They had me going for days! I was pissed beyond recognition. Of course, after a while I was laughing my ass off alongside them. Good joke. Cruel, but good.
My revenge: A CO2 fire extinguisher rigged to go off into his crotch when my buddy sat down to work.
Fun place to work. We did this kind of stuff all the time. Today I'd be afraid of getting sued. People have really thin skins these days.