angry chimps wielding croquet mallets. 
I feel your pain clavus, we had some slacker contractors mess with our roof (longer, much more involved story). Winter came and there was some rain. One day, HogansGoat comes by and says, "Hey, why is this particular wall damp?" Actually, I think his exact words were, "What the fuck is wrong with your wall?"
Turns out that the rainwater was leaking in, soaked that particular piece of no-longer-drywall. Fortunately, we had it fixed before it caused problems. There was an outlet at the bottom of the wall that probably would have caused some serious suck if it shorted.