Quick bug report:
When worshipping the nature lady and defeating plants when at 0 Piety, the hovering white tooltip thingy (the one at the numbers to show how much they change, like +2 or -4 at the HP/MP/... stat) says +15 Piety, while not actually increasing Piety. Punishment stays as normal.
How I found the bug:
Play and discover The Earthmother, defeat boss, have fun around in the dungeon, clicking multiple times on all the boons before exiting the dungeon until I had only a few Piety left. But now the dungeon exit was covered in plants, so I hacked my way through to the exit, killing all the plants and got major punishment for it. I don't really care at this point, since the boss is already dead.
I just thought to myself, how far can the negative corruption/... go and started killing other plants. Each time I killed a plant, I got major punishment and the Piety stat, while staying at 0, said it increased by 15.
Steam version of Desktop Dungeons, on OS X 10.9