A few moments from games that were memorable:
In one of the first games I DMed, I made a novice mistake of allowing the Savage Species Shadow in my game.
And it was optimized up the wazoo by the player.
However, it led to what is widely considered by my gaming group to be the best moment in any game of D&D.
He had some sort of ability that allowed him to deal acid damage, and the lord of the city they were in's chariot rolled down the street they were on, and the shadow stayed in the way, and dissolved an axle (which I allowed.) The chariot continued on, as did the players (the people not playing shadows, i.e. all of them but that one, came out of hiding behind some barrels for cover) and my narration:
Me, DMing wrote:"The street gradually starts to run downhill. After you have walked about 200 feet, you hear "AAAHHH!" *crash*"
Everybody present burst out laughing, and I was told that it was the best ad-hocing of anything that any of them had ever seen.
Later, the same shadow captured a slaver in his ship, disfigured him using the acid, and sold him as a slave to the lord of town (who was now in crutches, and reported that "his wagon mysteriously broke.")
The same player later decided to play a feral mineral warrior half-ogre half-minotaur gnome barbarian (as we both agreed that the shadow was game-breaking, and explained in-game that the shadow wanted to pursue solo killing projects), who was not broken, although a very powerful melee combatant (basically a Frenzied Berserker that could be killed, and didn't run risks of killing his friends, that had a Strength in the 50s, but an Int of 3 and a Cha of 1).
The party sorcerer once introduced him as "his Pokemon."
Later, they entered a house in the same city, and there was the silhouette of an attractive woman, who was behind a curtain in her quarters. The sorcerer decided to seduce her, in order to get some information out of her (they knew she was prominent in town) and rolled a 20 on his Diplomacy check (he also had maxed out ranks and a maxed out Cha). The woman, as it turned out, was a yuan-ti half-blood with a serpent's head, who fell wildly in love with him and begged him to marry her, offering him anything. He said that he would meet up with her later, after receiving her fortune.
The mutant gnome (his player and I call him the proto-Mulk, as it started a trend of his producing DMFs called "Mulks," apparently named after a Hulk parody in a bad episode of the Simpsons) decided that he wanted to join in on the action, and, horrifically, pulled down his pants.
It turned out that something was gnome-sized on an ogre-sized creature.
The yuan-ti lady ordered him to put his pants on and leave, of course, and soon enough everyone had, trying to forget the ordeal, even though they were a lot richer.
Strangely enough, this wasn't the last time someone in my gaming group would have a character pull down his pants (but more about that later.)
So eventually, the party made their way to the BBEG (nothing out of the ordinary, a necromancer lich of a few levels above them)'s tower, and made it to the highest room, where they saw nothing but a cat (actually the lich in disguise). The sorcerer said that he hated cats, and fired a spell off at it, breaking the illusion and causing a rather hilarious way of preventing an ambush. His player later stated that he didn't know that the cat was anything more than a cat, and it really was just that his character hated cats.
In a friend's game, another player (not the one who played the shadow and the gnome) played an ogre who, in the middle of combat with
some kappa fighters who wielded exotic weapons, decided to ejaculate on them.
No, really.
He did, although of course it didn't do anything besides gross everyone out, and it was up to the rest of the party to finish them off. After I killed the dual sai-wielding one with a fireball (which, in a reference to the old Turtles in Time arcade game, caused them to state "Forgot the sunscreen! We don't want his kind though,") the DM stated sadly, "Raphael was my favorite turtle."
Later the ogre went up to a woman guarding the entrance to a building, and tried to seduce her, which resulted in him getting punched in the balls. When he continued, he was captured (no one cared to rescue him) and we later found him castrated. We later found out what had happened to what he had lost, as this was an Oriental Adventures game, and it turned out that dried Ogre penis was considered powerful and valuable traditional medicine.
Needless to say, no one's character ever pulled their pants down in another one of any of our games again.
Nor did that player ever play a male character again.
Although, our characters did start selling ogre wang whenever we defeated an ogre in any game.
In a later game I ran, one of the players, a tiefling wizard, cast Tasha's Hideous Laughter on the BBEG, a devil who ended up rolling poorly on a Will save, and they sawed off his arms and legs, while he was still laughing.
I showed this as a depiction of what he looked like.
They later ended heading up to the same city where they had wreaked havoc in an earlier game, and by this point the ruler had found out what had happened to him, and they ended up hunting down the shadow (with my friend's permission, of course; it was actually his suggestion that we do that).
On the way there, they went around killing everyone in town (other than the Thieves' guild, who they were working with), and casting Speak with Dead to get info from them as to where they hid their valuables.
At one point another ogre PC (I don't know what it is with people playing ogres in our games, but this one never pulled down his pants, so it was OK) hit a bandit so hard (more than enough to kill him) that I ruled that he bashed the head off and over the wall of the city like a home run.
He also killed a leprechaun.
Green goo went flying everywhere.
In a 4e game (hey, I hadn't played it before), I played a warlock who used Intimidate to end an encounter with some kobolds, which, since nobody knew how the hell Intimidate was supposed to work, allowed me to get two kobold slaves.
Best. Reward. Ever.
And finally, my friend (the same one who played the shadow and the gnome) is running a Tome game, where I'm playing a quadruple kusari-gama wielding ghoul thri-kreen samurai (with a Dex of 36) who whores out on attacks of opportunity.
This is by far the most ridiculous game I've ever been involved in.
The game's premise is that all elves are inherently evil, and banded together when they realized that they could rule the surface world. Kraagik (the name of my character) was a leader of his people, who were slaughtered by the armies of an elven general named Megakill (one of the villains of the campaign). They were ancient enemies of the elves, and Kraagik himself was brought back from the dead as a ghoul by a wizard (Kraagik's Lord, who later turned out to be working with the elves, causing Kraagik to strike him down in rage). Tormented with the spirits of his people, who essentially functioned as his ancestors, Kraagik vowed to feast on the heart of Megakill.
On an aside, since the DM forgot to design a map for his world, he used a map of Middle-Earth, much to the dismay of the Lich Paragon (who used the method of destruction of his artifact phylactery as "the fires of Mount Doom," because there actually was a Mount Doom in this game.
The party, searching for a man named Flounder, as he supposedly knew a lot about the elves' plans and thus went into hiding in fear of retribution, got attacked by a Tyrannosaurus that charged headlong into a wall of force placed by the lich. The DM ruled that it died on impact, despite the player himself suggesting that it shouldn't.
It turned out to be Flounder's pet, and the lich's offer to bring it back as a zombie didn't seem to be much consolation, but he still eventually gave us the information we needed on Megakill's army.
There's also a running gag of us being attacked by giant constrictors. (Why does it have to be snakes?)
We spent much time in Bree, the last line of defense against the elves, which resulted in a lot of bad cheese puns (most notably, Megakill sent his advanced calzone golem (no, really, calzone golem, it's from some Wizards adventure) "Pizza the Hut" (a homage to Spaceballs, of course) and its follower Boba Feta, both of whom we soon dispatched of.)
Soon, we met with Megakill himself, who was coming to dispose of us personally. He was described as looking like Judge Doom (after he reveals he's a Toon) from "Who Framed Roger Rabbit" wearing the Dark Knight Joker's makeup and Glasgow smile, with pointed ears, wearing the Comedian (from Watchmen)'s leather armor, and carrying a Colossal crossbow (he was a Jester, and the DM forgot the weapon sizing rules in RoW, but we all agreed that it was so ridiculous that it was OK for him to be able to wield it. There was Rule 0, of course, but again, nobody minded.) His spells all had verbal components of bad puns. After some Who Framed Roger Rabbit and The Princess Bride references (Megakill: Remember me, Kraagik? When I killed your clan, I sounded just... like... THIS! Kraagik: My name is Kraagik. You killed my clan. Prepare to die.) Leaving the rest of the party to dispose of Megakill's shadows (he had the Necromantic feat to create incorporeal undead,) I disposed of Megakill, and did indeed eat his heart. However, he had a contingent Magic Mouth cast on his heart, and it insulted my ability to eat it properly. He similarly had one cast on his head, making bad puns as always, but I still kept it as a trophy (houserule: the DM gives action points to anything that makes him laugh, and that includes bad puns, and agreed that having Megakill's still talking head was a good way to do that).
Kraagik was about to kill himself, considering his quest over, but was approached by the spirits of his ancestors, saying that Megakill was only the subordinate of a more powerful elf general, Death By Nitrous Oxide. After returning to Bree, we found out that the mayor heard that Death By Nitrous Oxide was preparing an all-out attack on Bree, and we headed to their encampment. Most of them were low-level warriors that were taken out by the lich's cloudkill. When the lieutenants saw us surrounded by so many dead, they asked what happened. The lich replied, completely deadpan, "I farted." I couldn't help but laugh. After killing them, we used Speak with Dead to find out where Death by Nitrous Oxide was, and discovered that he had taken the form of the mayor's son!
Heading back to Bree, we found the mayor, who told us that his son was dead. This was true, but now Death by Nitrous Oxide had taken form of the mayor. He revealed his true form. Death by Nitrous Oxide was Huge sized, had a face "resembling a cross between an ape and a lizard," with a Mad-Eye Moody style giant googly eye and "steam shooting out of his pointed ears like an angry cartoon character, only constantly", two "Popeye" arms, and four thri-kreen arms, which were sewn on, taken from a legendary thri-kreen hero. The rest of his body resembled a huge, muscular elf's, and he wore naught but underwear, suspenders, a cape, a top hat, and a monocle over his non-crazy eye. He wielded four trees (game mechanically: ironwood greatswords) and a tower shield taller than himself. Death by Nitrous Oxide could survive in melee combat with me, and save-or-dies from the lich (none of the other PCs had even a chance to hurt him), and although we eventually whittled down his health to near-death, he teleported away. Kraagik threw Megakill's severed head at the ground in rage.
We leveled up, as his fleeing counted as a defeat, but we were also warned that Death by Nitrous Oxide would return, even more powerful next time (read: he leveled up too.)
OK, that was a lot longer than I expected.