These are from the annals of WebRPG; some of them older, but all of them hilarious and worth a read. If you have some hilarious PnP gaming stories of your own, share them here!
Cheers,
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1. Our party's wizardess was very pregant, and while we were in a wagon on our way into the city, her water broke, so we quickly spurred on the horses, and got to the nearest town in about 10 minutes.
We rushed her to the clinic, but the midwife was already delivering a baby.
After about an hour waiting the wizardess's baby decided it had waited long enough and came out. well, the dwarven fighter, a very impatient, yet chivalrous soul went into the delivery room to see what the hold up was. What he saw was a woman with a black shawl tied tightly over her eyes and head lying motionless on the floor, a stone statue of a woman holding the living breating baby, which had snakes for hair. The fighter, knowing a medusa when he saw one, promptly fired a crossbow bolt into the baby, killing it.
He then removed the shawl from the motionless woman, she as well was a medusa. She had died during the childbirth.
Well, the wizardess just happened to have a scroll with stone to flesh on it. So she read it and the midwife took care of the wizardess. After all that was done, the midwife asked the party what had happened to her and if it had something to do with the baby, the dwarven fighter spoke up with the memorable reply, "She had her mother's eyes."
2. "Once, my brother was running a campaign in a vast plain, and we were just trying to survive the dreaded "giant praire dogs". We've never seen the things before, and we went on our way to a town, when we encountered a troop of orcs! We had little time to react, because the fabled "giant praire dog" made his appearance, but we didn't have a miniature to use for the praire dog. The DM was prepared and launched his pet hamster across the board the group of figures! The hamster ran over the orcs and the party, dealing enough damage to them to kill the orcs and to leave us wounded! It tooks us five whole minutes to stop laughing and catch the hamster!"
(My own comment here...this one was pre-BG; but perhaps it really was a Giant Space Hamster?) 
3. "A brave but rather dumb Fighter Saw what he logicly assumed was a big Pile of jello in the hall. Rushing forward to take a big bite...Seconds later in a muffled voice "My Lips Are Stuck in the jello" Next went the hand then the other hand then one leg. The the muffled and paniced voice "Guys help I'm stuck in the jello" This was the fighters demise as the party rolled on the floor laughing and the gelatinous cube happily slurped up the fighter."
4. "In the heat of the final battle, the wizard of the group made a quick decision, yelled to the rest of the group to stand clear, and cast a chain lightning spell on the pool of water directly below the dragon. Just as the die hit the table, a large flash lit up the room from behind the recliner. One of my siblings, stumbling through the house in utter darkness, reset the circut breaker, only to reveal the family cat "Squirt" standing on the gametable, with its fur completely singed and smoking. After regaining our composure, and almost dialing 911 for multiple cases of busted guts, we checked the die roll, and to add to the ironocism, all five of the rolls were sixes, enough to finally finish off the poor sucker."
(Comment mine: I assume they meant the dragon, and not poor 'Squirt' )
5. "I was GMing an AD&D game where one of the party members (a thief) was stoned by a beholder. The beholder was defeated and the remaining characters hauled the statue back to town... That night, the party wizards began work with a local artificer to create a Wand of Stone to Flesh (this ended up taking many weeks of work during which the statue sat in their house). The fighter spent that first night drinking heavily in the local tavern... After a bit of "thought", he realized he hadn't done much with his hobby of painting lately. Putting two and two together to get five, he decided to paint the "dull gray" statue of his friend. (The player of the stoned character was, needless to say, not amused by this.)
Ultimately, the thief was restored, only to find himself covered with paint. He was furious! He turned to the party members and cried "What is all this paint?". The party members turned to the fighter, who blurted out "It was a spell component..?"
6. "OK, this was in a AD&D adventure and I was a PC. Our group of adventurers were hopelessly trapped in a town with little more than the shirts on our backs.
We wandered over to the Pit Fighting arena and our attention was caught by an enormously fat man with expensive looking jewerly hanging off of him.
This aroused a plan...
This little experience in the arena took us over 3 hours to play out (most of the time spent rolling on the floor laughing).
Our first plan was that my character would run by the man and yell "Free donuts by the west wall!" (the man was on the east wall, and like I said, very fat) then attack him as he was running for the wall. So I did that, and the man pulls out an arquebus and fires a quick shot in the air. "MOVE!" he yelled and immediately the entire crowd parted revealing a blank west wall... he didn't move an inch...
That plan failed...so we decided that one member would clumsily bump into the man and then we could use this as a distraction while we attack him. The member bumped into the man, but was hurled several feet back onto the hard stone floor (taking 3 points of damage) but the man didn't even notice the bump...
Another plan failed...so this time we decided to cause a distraction in the arena then attack the man while he went to check it out. It seems that several bails of hay were placed along the perimiter of the arena. So our archer fired a flaming arrow into one of the bails which in turn caught the entire perimiter on fire. The man goes to look at the commotion from the balcony and we attack. The group's theif jumped on his back and started hacking away at his double (and triple..) chins. The man lost his balance and broke through the railing, landing in the arena. The theif nearly died from the fall, but he managed to sneak away before the local militia decided to investigate the murder.
After this incident at the BEGINNING of the adventure, we could barely keep strait faces through the rest of the night!"
7. "Setting: AD&D, Forgotten Realms
Being a DM for more than 7 years, this was the most incredibly stupid thing I have ever had a player do.
A Paladin, Sampson Ramstead, found a white dragon figurine. While on the way back to the city he had the party's mage try to figure out what it could do since it was magical. The mage informed Sampson that the object had three functions, enabled by three different command words. For lack of nothing else, I informed him that the commands were Doe, Rae, and Me. Not knowing which does what, and what does wich, Sampson decided he would wait to test them at a later date. Upon entering the city and completing a very difficult adventure, Sampson and company went to a tavern. While sitting there, a freind of Sampsons who happened to be a powerful mage came over. Sampson told his freind about the figurine and pulled it from his pouch. While Sampson was holding onto the figurine, the mage asked him what it does. Being the up and mighty paladin, Sampson of course said he didn't know but it is supposed to do something when you say "Do, Rae, Me"! Well, low and behold a rather large White dragon erupted from the figurine, destryoing the small tavern and causing a major riot. Sampson, laying down on the floor with a dragon standing on his hand said, " well, at least I will be famous for something."
8. During the Time of Troubles in the Forgotten Realms magic was going awry. At the end of the AD&D module "Waterdeep" a fighter/mage PC on a roof turned around to find that the avatar of Myrkul, God of the Dead, was behind him. Reaching for his two swords the fighter/mage was dismayed to find that his two swords had been inadvertently turned to bunches of flowers by the spell he had cast a few minutes before. Holding the two bunches of flowers in his hands and watching Myrkul advance the fighter/mage mumbled.."I really hope Myrkul has hay fever!" (ps he escaped and survived..just).
9. I was playing in a game of Level 5 or so characters. On a quest to find a mercenary group that has repeatedly raided the town stealing from everyone to slaughter them, we came across a really powerful wizard bribed by the mercenaries. He casted Crystalbrittle on our swords so they shattered. So I picked up a stick from the ground and yelled to the wizard, "I WITH MY POINTY STICK WILL MILDLY ANNOY YOU!" I Then proceeded to poke the wizard excessively with the stick. It worked too! Being poked, the wizard could not concentrate on any spells, and the rest of my party huddled around the wizard and beat him to a pulp. The party got a lot of magical rings, including one that enchants your weapon to +2. So from then on, since we could not replace our weapons until we get back to town, I would shout out to our opponents, "I WITH MY MAGICALLY ENCHANTED POINTY STICK WILL MILDLY ANNOY YOU!" Miraculously we survived the quest, but I kept the stick through the entire campaign over any other weapon.
10. This is what happens when you mix paranoia with a lack of vocabulary. Remember, this story ACTUALLY HAPPENED!
The PC's were walking through a forest (I think) when they came to a large clearing. In the dead center of this clearing was a large gazebo.
PC- "Did it notice us?"
DM- "No...It's a gazebo."
PC- "I cast detect magic. Is it magical?"
DM- "No....It's..It's a gazebo."
PC- "I cast detect evil"
DM- "It's not evil. IT'S A GAZEBO!"
PC- "I throw a rock at it. Does it notice me now?"
DM- "NO! IT'S A GAZEBO!"
PC- "Fine, I attack it. Does it notice me now?"
DM- "NO!"
PC- "But it's a +4 Longsword!"
This goes on for another 5-10 minutes when the DM finally says,"FINE! You have angered the Gazebo gods. They animate the gazebo and it eats you! Role up a first level character!"