Two that occurred in my presence:
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First

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A bunch of serious role-players are trying out a new/young game master who is running a fantasy game in Role-Master (a.k.a. Tables "R" Us). At a dramatic moment in the story, one player (a foppish lordling with a rapier) is facing down the equivalent of a Urukai-Orc (an even match) on a bridge ...
Lordling: In a tired, dismissive voice announces "I thrust him casually through the throat"
GM: "Um, you can't just say that. You need to roll it"
Lordling: "Fine.", and promptly rolls an obscene critical, and repeats "I thrust him casually through the throat!"
GM: Looking at his tables, starts reading "Rapier damage critical... liver punctured, creatures dies in 1-3 rounds"
Lordling: The player gently forces the GM's book closed and says, "I thrust him casually through the throat".
GM: Finally, the light dawns. "You thrust him casually through the throat"
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Second

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A group of players and the GM are sitting around a table rolling up D&D 3rd ed. characters using one of the more the more stringent methods where stat rolls cannot be re-arranged to taste, and one of the players has a problem...
Player: "Umm, I'm trying to roll up a mage, but I've got a problem with my Constitution score."
GM: "What Is is?"
Player: "4" (one short of the worst possible in the game. You get negative hit points for this, and for a mage, this means that the common cold is a serious threat!)
GM: "Ouch. Well, since this is a more heroic-style game, trying rolling it again."
Player: Re-rolls. "4"
GM: "Okay, try using just 3 dice straight up" (usually, it is roll four and drop the lowest)
Player: Rolls. "4"
GM: "2 dice?". Other player start gathering around at this point
Player: Rolls. "4"
GM: Now just curious as hell, "1 die?"
Player: Rolls. "4"
GM: Not missing a beat, "Okay, looks like fate has decreed that your character's Constitution is a 4. As a consolation prize, here is a Ring of Regeneration instead of the other starting magic item I was considering for your character..."
-Bun