My personal favorite, that I read from another thread in a time gone by...
A party was fleeing from the Big Bad Evil Guy, who happened to be well-aware of the party illusionist's typical tricks. While crossing a mountain pass they went over a river of lava, and the illusionist stopped to cast a Silent Image of normal rock over the lava flow behind him. He then waited for the villain to show up, and said to him something to the tune of, "Now for a taste of my TRUE power!". He then went through the verbal and somatic components of Silent Image without actually casting it, and as a free action dismissed the spell that was in effect. The GM frowned, and had the villain order his hordes of troll warriors forward...