These are old stories but they're still pretty funny*.
The Tale of Eric and the Dread Gazebo (link)
...In the early seventies, Ed Whitchurch ran "his game," and one of the participants was Eric Sorenson. Eric plays something like a computer. When he games he methodically considers each possibility before choosing his preferred option. If given time, he will invariably pick the optimal solution. It has been known to take weeks. He is otherwise, in all respects, a superior gamer.
Eric was playing a Neutral Paladin in Ed's game. He was on some lord's lands when the following exchange occurred:
ED: You see a well groomed garden. In the middle, on a small hill, you see a gazebo.
ERIC: A gazebo? What color is it?
ED: (Pause) It's white, Eric.
ERIC: How far away is it?
ED: About 50 yards.
ERIC: How big is it?
ED: (Pause) It's about 30 ft across, 15 ft high, with a pointed top.
ERIC: I use my sword to detect good on it.
ED: It's not good, Eric. It's a gazebo.
ERIC: (Pause) I call out to it.
ED: It won't answer. It's a gazebo.
ERIC: (Pause) I sheathe my sword and draw my bow and arrows. Does it respond in any way?
ED: No, Eric, it's a gazebo!
ERIC: I shoot it with my bow (roll to hit). What happened?
ED: There is now a gazebo with an arrow sticking out of it.
ERIC: (Pause) Wasn't it wounded?
ED: OF COURSE NOT, ERIC! IT'S A GAZEBO!
ERIC: (Whimper) But that was a +3 arrow!
ED: It's a gazebo, Eric, a GAZEBO! If you really want to try to destroy it, you could try to chop it with an axe, I suppose, or you could try to burn it, but I don't know why anybody would even try. It's a @#$%!! gazebo!
ERIC: (Long pause. He has no axe or fire spells.) I run away.
ED: (Thoroughly frustrated) It's too late. You've awakened the gazebo. It catches you and eats you.
ERIC: (Reaching for his dice) Maybe I'll roll up a fire-using mage so I can avenge my Paladin.
At this point, the increasingly amused fellow party members restored a modicum of order by explaining to Eric what a gazebo is. Thus ends the tale of Eric and the Dread Gazebo. It could have been worse; at least the gazebo wasn't on a grassy gnoll.
The Head of Vecna (link)
Many years ago (back when we all were still playing D&D), I ran a game where I pitted two groups against each other.
Several members of Group One came up with the idea of luring Group Two into a trap. You remember the Hand of Vecna and the Eye of Vecna that were artifacts in the old D&D world where if you cut off your hand (or your eye) and replaced it with the Hand of Vecna (or the Eye) you'd get new awesome powers? Well, Group One thought up The Head of Vecna.
Group One spread rumors all over the countryside (even paying Bards to spread the word about this artifact rumored to exist nearby). They even went so far as to get a real head and place it under some weak traps to help with the illusion. Unfortunately, they forgot to let ALL the members of their group in on the secret plan (I suspect it was because they didn't want the Druid to get caught and tell the enemy about this trap of theirs, or maybe because they didn't want him messing with things).
The Druid in group One heard about this new artifact and went off in search of it himself (I believe to help prove himself to the party members...) Well, after much trial and tribulation, he found it; deactivated (or set off) all the traps; and took his "prize" off into the woods for examination. He discovered that it did not radiate magic (a well known trait of artifacts) and smiled gleefully.
I wasn't really worried since he was alone and I knew that there was no way he could CUT HIS OWN HEAD OFF. Alas I was mistaken as the Druid promptly summoned some carnivorous apes and instructed them to use his own scimitar and cut his head off (and of course quickly replacing it with the Head of Vecna...)
Some time later, Group one decided to find the Druid and to check on the trap. They found the headless body (and the two heads) and realized that they had erred in their plan (besides laughing at the character who had played the Druid)...The Head of Vecna still had BOTH eyes! They corrected this mistake and reset their traps and the Head for it's real intended victims...
Group Two, by this time, had heard of the powerful artifact and decided that it bore investigating since, if true, they could use it to destroy Group One. After much trial and tribulation, they found the resting place of The Head of Vecna! The were particularly impressed with the cunning traps surrounding the site (one almost missed his save against the weakest poison known to man). They recovered the Head and made off to a safe area.
Group Two actually CAME TO BLOWS (several rounds of fighting) against each other argueing over WHO WOULD GET THEIR HEAD CUT OFF! Several greedy players had to be hurt and restrained before it was decided who would be the recipient of the great powers bestowed by the Head... The magician was selected and one of them promptly cut his head off. As the player was lifting The Head of Vecna to emplace it on it's new body, another argument broke out and they spent several minutes shouting and yelling. Then, finally, they put the Head onto the character.
Well, of course, the Head simply fell off the lifeless body. All members of Group Two began yelling and screaming at each other (and at me) and then, on their own, decided that they had let too much time pass between cutting off the head of a hopeful recipient and put the Head of Vecna onto the body.
SO THEY DID IT AGAIN!... [killing another PC]
In closing, it should be said that I never even cracked a smile as all this was going on. After the second PC was slaughtered, I had to give in (my side was hurting)...
And Group Two blamed ME for all of that...
*If you're a big geek.