This party had just entered a cavern that the DM had described as " an expansive, open-ceilinged cavern", and had spotted it's occupant: an adult red dragon, one that was clearly not happy to see them. The players, who were all around 5th level at the time, were supposed to have ended up doing a quest for him. That's when this player had a bright idea...
Player: Hey, how big is the tunnel we came in through?
DM: About 4' wide by 8' high.
Player: Are there any other exits in this cavern?
DM: No. Why?
Player (smiling): Alright, I draw my sword.
DM and other players: WHAT?
Player: There's no way an adult red dragon could have fit through that tunnel. It has to be an illusion!
DM (motioning for the others to be quiet): So why are you drawing your sword?
Player: If I take a swing at it, it should dispel the illusion.
DM: Okay, you draw your sword. (trying to give the player a subtle hint) You idly notice raindrops bouncing off of the blade.
Player (apparenly not getting it): Alright, now I rush at it!
DM (shrugs): Okay, you rush at the illusionary dragon, which swings an illusionary claw at you and illusionarily rips your head off. Your character sheet, please.
Player: WHAT? That can't happen! It was an illusion! There was NO WAY he could fit in here.
DM: Sure there is. He came in from above.
Player: Above?
DM: Yes, above. I told you it was an open-ceilinged cavern, right?
Player (after a long pause): It was?
DM (while the others nod their heads): Yep.
Player: Nooooo....
The dragon ended up letting the others go, since it was still laughing too hard to do anything else with them.