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.
Had basically the same thing happen to me in the last apartment I lived in. Upstairs neighbors put a crapload of potato peelings down their disposal which plugged up beneath my sink, resulting in dirty water and potato peelings all over my kitchen floor.
Had to run up and tell them to stop putting crap down their sink until the apartment management could get it unplugged. Naturally 30 minutes later they started putting crap down their disposal again... "Oh, yeah... we forgot."
