>Someone in our community (Dan Freeman, maybe) used to talk about some corruption that only occurred at a certain time of day. After a long time, they figured out it was when a vacuum cleaner was being run or something like that.

We had bigger issues than that. The city used to be industrial, and most of our customers were factories, with PCs installed in or near production lines. Sometimes we had data corruptions which magically vanished once they moved the coax cables away from the power lines - because those have powered heavy electric motors of a few hundred kilowatts. Every time one of those would start or stop, extra network traffic... of sorts... would occur. In other cases, the network cable would go through a workshop and somehow some arc welding would happen nearby.
Among the weirdest was the case when the cable spanned the distance between two buildings, and they said it was hung high enough. Well, high enough for most of the cases, until they drove an even taller truck there...
The common cause in the early nineties was bad extension chords, where we actually measured 103V between zero and ground (!). I could sense the voltage if I'd just touch the terminator or BNC plug. But these were at least easy to spot, measure and fix.