A toilet almost bought down a cargo bird a few years ago. The ground crew put too many of the detergent balls into the toilet (makes the water blue and kills smell, etc). The balls foamed up too much and the foam mixture overflowed the holding tank and the water/foam mixture dripped into the avionics bay and started a fire. The crew was able to make an emergency landing.
I have to morbidly wonder, did the ground crew get fired, demoted, or Union Protected? :)