Packet-forging exploits have been used for some time to alter the damage players deal to monsters or receive from them. Combined with Meso Guard, hackers were able to fool the game into hitting them for -2,147,483,648 damage (the largest negative value a signed 32-bit integer can hold). Since the damage was a negative value, the Meso Guard skill then granted the player over two billion mesos. Hacking players used their ill-gotten gains to buy up practically everything in players' market stalls, massively inflating the game's economy by pouring untold billions into the hands of random players.
Nexon attempted to patch the exploit last night and responded by permanently trade-banning countless accounts. Reports from players indicate that the exploit is still possible and that many of the offending accounts were not banned. To make matters worse, a large number of legitimate players who were traded ill-gotten currency were also allegedly trade-banned. The follow-up to another meso exploit found last year was similarly lackluster, with no server rollback performed in either instance.
[Update: Scott Yoo from Nexon Europe got in touch with us to provide an update on the story. He had the following to say: "[W]e did 'trade-ban' suspected accounts temporarily in order to conduct an internal investigation but we also had to make sure no-more illicitly gained Mesos (in-game currency) are going out to the in-game market and contained in the meanwhile. We do admit that a certain amount of legitimate players can be included in the trade banned list and that's what we are going to reveal through the investigation carefully. [...] The reason why we didn't perform a rollback on our servers was because we detected the exploit too late which may make the players lose even more of their gains if we were to turn everything back. Another announcement that includes more details on this incident will go up live shortly (either today or tomorrow as the latest)."]