I've been playing 3.x edition Dungeons and Dragons for a long time now - pretty much ever since third edition came out, in one form or another. After you play for so long, you begin to just remember the vast vast majority of the rules, and I've very recently discovered (if you couldn't infer from the title of the post) that this is precisely where mistakes get committed to memory and risk never being corrected.
- One of my first groups assured me wholeheartedly that ranged weapons acted as if they had point blank shot if fired from within 10 feet of an opponent, even if the character firing it doesn't have the feat.
- For a long time, due to information erroneously gifting the Masterwork Bandoleer (Faerun Campaign Setting) the ability to draw things contained within it as a free action, my friends and I thought potions were the best things ever.
- An old friend of mine, when I had first gotten into the myriad of ways one could play D&D online, was happily teaching his noobs that "monks were awesome" because they could "flurry of blows as a standard action".
- I heard an interesting story about how a player convinced his DMs that since - according to the fluff about sorcerers - he had dragon's blood him him, he qualified for all the feats that requiring some sort of draconic heritage. Win.
- A friend of mine, Marz, made a Kenku Ranger with the variant Distracting Attack. If a Distracting attack hits, the enemy is considered flanked for the purposes of an ally's attack. He worked days on it, and then the DM promptly informed him he didn't count as his own ally right before the character got to see any action.
- Someone I played with briefly created a "totally ridiculously cool character" using the "Gestalt Rules". Except he thought it was just some well hidden rules that anyone could use!