I was working as the IT director at a private school with a buddy of mine. We had an issue where every morning, a segment of the network would go down and we couldn't figure out what was going on. After finally tracing it to a distant switch, we ran diagnostics on it (remotely) and found nothing inherently wrong with it but we were able to trace the outages and saw that they were frequently and almost regularly happening within a few minutes plus or minus 7:30 AM. So we decided to come in before that and take a look at the switch in person.
The switch was located in an 'attic space' that was being used for the theatre department and when we got there we saw that they had moved the switch and sat it on top of a mini fridge with the cabling draped over the back. You can probably imagine what the cause was as well as we did. When the theater department director would arrive in the morning, first thing he did was make a cup of coffee. Then he'd open the fridge to grab some cream, leaving the fridge door open... as soon as the compressor would turn on, it injected just the right amount of EM interference to screw with the switch.
Needless to say, we relocated the switch, moved the fridge to the other side of the room and warned the staff not to relocate equipment without consulting us first.