In the episode where they are all about to leave to go on a trip because Phoebe was jealous at missing the London trip due to being pregnant (I forget where they are going - Atlantic City, maybe?). Anyway, just as they leave, Phoebe's waters break and the 3 boys all jump back in horror. When you next see Phoebe, there is no sign of wetness on her or the floor. Her dress and/or the floor would be pretty wet, especially since it seemed to be quite bad given the boys reaction.