Hm, how to give enough background info for "you had to have been there" war stories....
In a 1E game we played, one of the players forgot to write down his thieving scores before beginning play; this after repeated admonitions by the "detail-oriented" DM to "make sure you've got *everything* written down." This was a one-shot, kind of no-holds-barred when it came to character creation, so the character was in fact a duelist/thief, intended for an Errol Flynn-ish stereotype. (Duelist NPC class from Dragon Mag, or something.) The character has forever after been referred to as the "duelist/schmuck." (The DM relented and permitted the character thieving scores, just not adjusted for his 19 or 20 DEX.)
At a con game (still 1E), one of the DM's buddies was hanging around waiting for an open slot. There was one, and the fellow sat down and pulled out a character sheet. Without batting an eye, the DM asked, "you're playing your mooth?" Yeah, you read that right. M-U/Th, for Magic-User/Thief. Mooth. We haven't seen either of these fine gentlemen since the late 80s, but you might say that one stuck with us. "Wizrog" just doesn't have the same charm, does it?
A skit that aired on the Dr. Demento show led to a mage and his fighter bodyguard named, unapologetically, "Mr. Wizard and his little friend Timmy." The Spelljammer was never the same again.
I *think* this one happened to a friend of mine rather than it being an urban legend. In a Dragonlance game, one of the players had read the books and knew when the wicker (i.e., fake) dragon appeared. Confidently proclaimed the fact to the other players, too. So when the amazed (i.e., frustrated) DM pulled out the MM and decided it was a not-wicker (i.e., really big and really real) dragon, the players were not amused (i.e., nearly had all their characters killed). "Oh, I've read this" is still enough to make that particular DM cringe a bit.
In a 2E game, the characters ran across a great wyrm shadow dragon. They could have left it alone, but they were so into the battle plan that they managed to scare the dragon away (right through a blade barrier, no less). The pursuer (a flying mage with a magic staff in each hand; think F-16) wound up losing the dragon, but not to worry: upon rejoining the party, he asked the psionicist where the dragon was. A piddling number of PSPs and a few minutes of running later, the party bashed open the door to the dragon's hiding place and finished the job. I started rolling dice. "What now?" asked the psionicist's player. "Hang on," I said. "He's rolling up the dragon's treasure! We weren't supposed to kill it!" exulted the mage's player. It was an impressive haul, for a Ravenloft module, and they really *weren't* supposed to kill the dragon. "Are we supposed to kill this one?" has lasted oh these many years.