At a club I worked at, the power would sometimes cut out when it had rained earlier. I don't mean the PA or FOh cut out, the ground fault switch for the entire panel tripped. After a couple years of this, it stopped at the same an installed light for some lounge seating element failed to come back on after the previous panel reset. When the light was investigated it was found that the power cable outer jacket had been stripped off in installation and patrons movement on the seating element had pinched the cable conductors to a steel frame which was part of the building, which happened to leak the ever slightest bit of water from the roof during heavy storms and like the water of the tao, find the place it wants to.
At a hotel I sometimes freelanced at, one of the wireless mics would randomly pop. Not often enough for the customers to notice, but drove me fucking crazy. I only worked there every few months and nobody else was bothered enough to try and track it down. As we were doing out setup once for a conference, it popped again while we had plenty of time to investigate. Eventually found it was the xlr output on a patch panel. Since the AV company was a bit lazyand the receiver lived in the rack, nothing ever happened to the panel and a short mic cable became the new patch.