On first visit (before you have found the secret stairs):
You start to ascend the staircase, but only get about halfway up before the stairs collapse, depositing you in a crumpled heap on the floor. Who left that crumpled heap there?
If breaking the stairs takes you down to 0 HP, you will be beaten up for 4 turns.
Attempting to climb the stairs before you have found the secret stairs will cause the stairs to break and the Stairs Up location to be replaced with Broken Stairs.