I remember the very first time a new player joined our group of experienced D&D'ers. One of them had told him something about trapped candles that shoot magic fire at whoever passes in front of them.
So, during the adventure, I (GM) am describing a narrow hallway, completely oblivious to the conversation mentioned above, and tell them that there are a pair of red candles.
Immediately, our new player goes: "I know this! Those are those dangerous candles, aren't they?" He proceeds to cut them to pieces and grins at me in triumph over his cleverness.
Me: " Ok. You're now standing in the dark."
Ever since that day, I always made sure there was at least 1 room in the enemy stronghold that had candles