I was out with a band once at my home town's fall festival, setup and ready to headline the local bar's beer garden. A local teen band was opening so I left a helper in charge and went across the creek to get some carnival food. Wasn't gone 10 minutes when a heavy thunderstorm came out of nowhere! I thought, oh, no big deal, I had a 10x10 tent over the electronic stuff, a hired tent over the stage, and a ton of tarps too.
Got back and found out that my helper freaked out and put tarps over the speakers (which are basically rainproof, we let them get rained on dozens of times a year) but he somehow failed to tarp a $3000 digital mixing console and a $800 light controller.
It took hours with a hair dryer to get all the water out. Fortunately for the helper all the stuff still worked and I had backup equipment in the trailer that day. But man, the look on his face when I walked back over and he had to tell me he let thousands of dollars of electronics get poured on, was priceless.