Ultima Online's devs once identified item-dupers en masse, then burned all their houses and stores to rubble

pcgamer.com:

Ultima Online is one of the longest-running and most influential MMOGs out there, and is currently celebrating 25 years. (There's even a special shield to mark the occasion.) Such a prestigious anniversary has inspired some of the talent behind the game to reminisce about their time on the game, including programmer and game designer Tim Cotten. His story is an absolute doozy.

Cotten writes about an omnipresent gremlin for gaming, and particularly massively multiplayer games like Ultima Online: item duplicating, or duping. He first encountered the problem as a player of the game, way back in the heady launch year of 1997, when he witnessed two players dropping chests at a particular area of the map and excitedly jabbering "“it worked! omgz!”

«Yup,» writes Cotten, «they had managed to, as they excitedly bragged, figure out a trick to drop a chest on one side of the 'laggy patch' while trying to pick-it-up/hand-it-over to the other player as they were both crossing from one side to the other and now each of them had a copy of the same chest: and its contents.»

The post is titled 'That Time We Burned Down Players’ Houses in Ultima Online' and chronicles part of Cotten's personal journey from poacher to gamekeeper, as what he'd once witnessed from the outside he later, as a member of the dev team, decided to combat.

The whole thing is worth a read, but I'm going to skip over Cotten's technical explanation of how Ultima Online generated its map and kept track of player movements (the tl; dr version is 'ingeniously'), and how he eventually worked out a way to identify duped items and the players who had them. You can read the full post here.

The important point is that Cotten implemented a global hash registry on Ultima