The minidungeon in Sleepywood, Ant Tunnel II, where you can fight Horny Mushrooms and Zombie Mushrooms doesn't allow you to use any kind of potions after entering. Neither do the monsters drop anything, allthough they still give EXP.
It's built into the map that you cannot use pots there so technically it is meant to be like that. I don't know if it is possible to fix this without wz edits.