thedungeondelver
Adventurer
Hell this goes all the way back to at least AD&D; and even if it wasn't written out in the rules before it was at least implicit. If the players, for example, are using flaming oil, then intelligent monsters will get their hands on some and reciprocate! Further, witch-doctors (basically cleric/magic-users who could cast certain 4th level spells) should be doing unto the players as well.
You're cheating your players if you aren't keeping them on their toes at all times.
You're cheating your players if you aren't keeping them on their toes at all times.