Brad's story reminds me of one of my own. I'll try to be brief.
At one of our parties at the last flat in which we lived, we warned the people downstairs that there would be a lot of people and noise, so they left for the evening.
My wife and friends were making snacks in the kitchen, and managed to clog the drain. A friend of mine plunged the drain, and everything was working fine, or so we thought.
When the downstairs neighbors came home, they saw water running out of their flat under the entry door even before they got inside. My friend had plunged the blockage below the level of the neighbors' drain, and all of the water we were running was backing up from there and overflowing his sin, flooding their kitchen and hall.