I guess it is some kind of bug of the new ability of zombies to climb over obstacles. Buildings are seen as low abstacles and Zombies can climb onto them?

Had the same idea about this. It should be supported in future. Its really senseless to not find a stone when the ground is showing thousands of stones.

The description given by Haos still works in current build 0.59 (stable) and also in 0.60 (exp).

This - i would call it bug - is still working and should be fixed.
Spamming F when drinking at a well and pond gets you hydrated/stuffed in a few seconds.