Couple of years back, LHR - MEL via Singapore. Quite bumpy so seatbelt sign had been on for hours, no drinks service. Finally lights went off, captain announcement that all should be smooth for now. Trolleys came out, and a nice can of beer duly poured into the glass. And then it was just like in the cartoons: plane dropped a mile (i.e. about 20ft), beer hovered in mid air for a while, then plane (and my lap) came back up to meet the beer. Judging by the screams, and then laughter, everyone was in exactly the same (damp) position.