Wasps...
Some species like to make their hives in the heat sink fins of outdoor electronics such as VFDs. We lost a 350HP supply pump VFD once, which caused a series of cascading problems that shut down the entire plant. Turned out that wasps had been using the heat sinks for their nests and completely blocked the air flow, so the VFD tripped on over temp, repeatedly, without the operator finding the problem, so he kept resetting it. Eventually the transistors blew up and in replacing the VFD, we found the wasp nest; the hard way...