many years ago, still at my parents' house, we had a leak from a pipe upstairs... A LOT of water, to be honest. well, some day later the bathroom ceiling went down. a piece of rock actually broke the WC in two halves. the same damn WC i was sitting on 10 minutes before! after that, until someone came to fix things up, we had a sort of a Brazil (Gilliam's movie, nothing to do with Fernando) effect, we could look in the flat upstairs through a 2 meters hole!
so don't overlook cracks in the ceiling. ever.