The first time I did anything D&D related, the DM was also new and probably should've known better than to let me turn an entire underground lake into holy water, thus dissolving the boss zombie he expected us to spend a couple of sessions fighting.

"So, there's a horde of weak undead and one gigantic one standing in some flooded ruins at the center of the cave..."
"I bless the lake!"
"You wha...? ...and they're all dead. Congrats"

Pic by Knucklebone.