As an example, I put a special troll in one of my games that was not only immune to fire, but would cause flames cast at them to blow up in a radius around them. The players saw the troll, ignored my telegraphing, didn't take the necessary steps to do verify their assumptions, and the wizard (who even had those skills!) blasted it with a burning hands spell. The resulting inferno (which happened the next round too due to a burning web spell) jacked up the party and dropped an important NPC who they now had to scramble to save. Suffice it to say, they check now, years later.