Flushing Toilet, Screaming Shower - TV Tropes
Bob flushes the toilet while Alice is in the shower. Alice screams and curses, unprepared for the sudden increase in water temperature. Or perhaps Alice turns on the washing machine while Bob is in the shower, causing Bob to experience a precipitous drop in water temperature. Can be done deliberately or by accident, but the result is still funny — provided it's not happening to you.