Now, in Asheron's Call, the world is huge. There are hundreds, if not thousands of vendors. And three of these vendors were set to sell their goods in stacks of up to 1000. Unfortunately, Cost of Pyreal Scrab * 1000 > 2^31, which wrapped. I can't remember if you either just got the goods for free (which you then sold back for huge profit), or if you actually got paid to take these things. Either way, overnight, the economy was destroyed. The entire game state had to be reset from backups; a dreaded rollback. Worse, the developer took a few days to do this.
Trust me, out of all the customers whose data you don't want to muss with, it's hardcore MMORPG players. Even though I was just a player, I can still remember the outrage all these years later. It taught me to always use appropriate types for objects with "value", and I've never accidentally used signed or floating point storage for currency again.