Casting Fireball in both a dead forest to kill off the feindish-treants, then casting it in an all wooden, makeshift fortress, while in the heart of it to kill the beserkers right in front of them.
A noob shot a lightning bolt at a blue dragon, making it very annoyed, but before hand doing it underwater (same player, different character).
A Cleric of Nerull blostering the zombies BEFORE trying to take control of them.
The list goes on. Needless to say, he learned that realism is a part of my game. He recently made a list of reasons not to fight an iron in meelee combat to go along with the previous elemental notes he took. This is all the same guy, he took out my need for an NPC to lighten the mood at the table, now we just reflect on his past mistakes.