Setup: The party had ventured to a ruin in the swamp where kobolds had been disappearing to. Inside they meet someone named Sol, a ghastly looking humanoid identifying as the embodiment of fear. I’m the GM.

Myself: “Sol narrows his eyes, stammering over his words as he accepts the proposal.”
Player: “Ha! I made Fear stammer! I am God!” followed by him writing down God on his sheet.

“But if I shift to my werewolf form I won’t be able to use my gun – I’ll be defenseless!”
“Yes, you will go from being a small woman with a small gun to being a 400 pound mass of muscle with razor sharp claws, and teeth – utterly defenseless.”

Setup: Playing some game of Starfleet Battles in the middle of a combat where the players can’t see the approaching enemies.

Player: “Let’s just pop out there and see what they’ve got…”

As I recall, he never had a chance to finish the thought. And it haunted him in every game/campaign we had after that….

Several from the old and current campaigns:

1. Optic Fibre
Our characters were in 1st edition Exalted setting, and tromping through Chiaroscuro, famous for the fact that everything is made out of glass. Including the sewers, where we were fighting against some wyld-warped nasties.

My Day-castle Exalted flared up at full strength, since we burned through essence points in this big fight. The GM began to wonder if he should start making some random checks whether people will notice this and investigate.

I protested, “I’m not turning the entire sewer into optic fibre!”

2. Diagnostics Tool
My D&D3.0e wizard PC had at one point a quarterstaff which was bane vs. evil outsiders. You can guess how it got applied…

3. Going to Chinatown
In one of the Vampire: the Masquerade campaigns a (new) player’s character was warned that no one, absolutely no one should go to Chinatown, and right now the entire city is in low-profile mode in order to maintain the Masquerade. And absolutely no going to Chinatown, ‘kay?

Five minutes later the player decided that he wanted to get a sword, went to Chinatown, and got his ass kicked.

“Going to Chinatown” ever since meant “doing what you were told not to do, and suffering the consequences of it”.

4. Aggressively
A dwarven fighter (AD&D 1e) attacked a bunch of monsters that were minding their own business and not particularly bothering anyone, however they were capable of whooping the party’s ass and make it a tight fight. After the bloody business was over…
“Why did you attack them?” the rest of the party asked.
“They… they were breathing aggressively!” the dwarf replied.

5. Corellon’s golden baptism
…what one cleric of Corellon in the aforementioned AD&D campaign did to a shrine dedicated to evil god Moander.

6. I bought this one from the market
In Middle-earth campaign we had a shifty wizard who had a dishonest streak mile wide. In a forgotten underground library we barely managed to overwhelm a lich, and us being adventurers, we started to loot it bare. He found a dagger, and pocketed it, without letting the rest of the party know.

Later, he went to consult our party’s much-higher level wizard regarding the nature of the item.
“Nice dagger.” the other PC said. “Where did you get it?”
“Oh, I bought it from the market.” (Note: it had been about a month we had been anywhere near civilization.)
Meanwhile, the other PC cast a spell that delved into the nature of the item, the GM slipped the item info on a piece of paper; it turned out that it was cursed, with Firenerves effect attacking the user.
“Did you pay much for it?” he asked.
“Oh, uh, not much, not much.” the shifty one stammered.
“Good, because it is cursed.”

The same shifty PC laid his greedy mitts on a magical necklace later in the campaign, put it on (to avoid yet another being laughed at episode)… and it was cursed as well. His skin fell off, and he had no choice but to fume and reveal what he had done.

7. Bardello
The nickname for a bard school one of our PCs started in a D&D campaign. The said PC worshiped Sharess, goddess of sensuality and sexuality…

8. Druids breed naturally
Magic can cause all sorts of mishaps, sometimes rather quirky ones. Our D&D 3.5e druid had an overprotective mother, who acquired a flesh sample of the said druid, as a “backup measure, so her dear mother won’t lose her ever again”; the said mother had access to powerful cloning spells of her own design.

What the druid did not disclose to her mother was the fact that at the time the clone sample was taken, she was pregnant with her first child. The DM had a bit of a puzzle in here, and eventually with the cooperation of the player interpreted that yes, the druid will return in that very same condition.

This meant that the druid gave birth for the first time twice, to twins that were over a year and half apart in age.

When the rest of the party found out about the situation, the bard (who worshiped Sharess, mentioned above) scoffed “Yeah, sure, she calls me a perv, while druids are all for natural breeding and childbirth…”

(By the way, the second kid ended up as a Spellfire wielder.)

Players track a murderer to a local tavern (okay a tea house). On first glance they don’t see him, so one player (viking-style warrior) approaches the guy behind the bar …

Player: Do you have beer?
Barkeeper: No
Player: Do you have booze?
Barkeeper: No
Player: Do you have wine?
Barkeeper: No
Player: Do you at least have a backdoor?

Describing an empty room to the party:

“The dust is spotless”

3ed ed. D&D
chars find a giant demon statue with rubies as eyes.
me: “you cannot get those out just with your bare hands.”
player 1: “ok, I need a dagger, hey, do you have a dagger?”
player 2: “yeah, but…”
player 1: “doesn’t matter, I use the dagger for prying out the rubies.”
me: “ok, you now have two rubies as big as your fists.”
player 1: “I give back the dagger.”
player 2: “oh no you don’t.”
player 1: “what?”
player 2: “that was the cursed dagger I got three rooms back. You know, the one you can’t let go off once you wield it?”
player 1: “FFFFFFFFFUUUUU–“

Heh,,, it reminds me ”famous last words”
– You dirty wizard, You shall taste my sword now!
Puuufff