Dust across PCB tracks and component legs, can become conductive in hi humidity and cause trouble.
Just in the last few days in Sydney we had the worst storms ever recorded damp everywhere in the house.
I had I thought my computer motherboard had bit the "dust" as it would not turn on even with a blade shorting out the turn on pins. Checked everything even changed the powersupply.
I saw a layer of dust across some of the tracks and swept it clean, then she turned on while I was sweeping it off, and it been fine ever since.
Cheers George