Okay, here goes...
Back in 2e, I played a gnome wizard with a god complex. Party encounters a very, very large black dragon and the dragon asked us what we were thinking when we entered his cave as he casually prepared to eat us.
I don't remember the full conversation, but at some point my gnome said "Alright, listen here, worm..." (bad pun, he knew) and the dragon went ballistic. I rolled high on initiative and managed to go first, so I cast resilient sphere and managed to get myself, the paladin, and cleric inside. The ranger... not so much.
So the dragon is rather upset with me, and is treating the resilient sphere like a cat toy and we are taking damage from being tossed about inside and I am beginning to think I made a mistake.
Ranger remembers I had identified an arrow for him earlier as an "arrow of dragon slaying", so he pulls out his bow, yells "DIE &#$^er!!!" and shoots the dragon, critically hitting and dragon was his favored enemy. He rolls damage and notices the dragon is still quite alive, who then turns around slowly and glares at the ranger. Ranger realizes something went very wrong, and takes off running the other direction with his boots of speed. Dragon leaves the "cat toy" alone long enough for me to dismiss it and then quickly cast teleport, taking the cleric and paladin with me to safety. Ranger eventually finds a tunnel small enough and escapes after the chase of a lifetime.
We all meet up in the little town not too far away and are sitting at the inn, when the ranger says "you know that arrow you gave me? It didn't work!". I casually said "I never said it was a BLACK dragon slaying arrow..."
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Same campaign a session or two later.
Party is deep in the underdark and we have been encountering these mushroom things that are annoying, but plentiful. We have grown completely tired of even having to roll initiative to fight these things.
We come to a cavern and things look slightly off. There is a large patch of mushrooms, but one stands out. Before my gnome could say anything, the paladin says "I GOT THIS!" and charges into the mushrooms. Well, the one in the center had some long tentacles and rolled to hit the paladin and critically hit. DM says "Okay, the tentacle hits you in the right arm, and the arm immediately withers away and falls off".
The look on the paladin players face, I'll never forget :)
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Last one for now.
Same player as the paladin, but different campagin. We are still somewhat low level and are camped out on a hill in the middle of nowhere. DM rolls for random encounters and lets us know that a large group of bugbears comes walking up the side of the hill, and is now standing 40 or so feet away at the very edge of our light and look like they are about to start a fight.
Paladin (again playing paladin) stands up and says "HAIL, bugbear travelers!"
Which has since become one of our groups catch phrases :P