“Oh I would never dream of assuming I knew all of Hogwarts’ secrets, Igor,” said Dumbledore amicably.  “Only this morning, for instance, I took a wrong turning on the way to the bathroom and found myself in a beautifully proportioned room I have never seen before, containing a really rather magnificent collection of chamber pots.  When I went back to investigate more closely, I discovered that the room had vanished.  But I must keep an eye out for it.  Possibly it is only accessible at five-thirty in the morning.  Or it may only appear at the quarter moon — or when the seeker has an exceptionally full bladder.”

Harry snorted into his plate of goulash.  Percy frowned, but Harry could have sworn Dumbledore had given him a very small wink.

Harry Potter and the Goblet of Fire – J.K. Rowling