I really think that the story (almost sounds like a fake tale similar to creepy pastas) of Px is the most curious and problematic thing that ever happened in a MMORPG.
The name of the character with a particular bug was Px. Weirdly enough, his name was that back in the early 2000's (you were allowed to own such names back them). In short, he discovered that typing "ae" (yeah, ae only) in the chat of Tibia would deliver to him random username and passwords (!!!!).
Yeah, the case is an absurd itself, but it really happened. There is a lot of discussion about this
The guy who had this char told everything about it here. "ae" is a common thing to say between the brazilian community, but somehow the word ae posted by the char px was a combination that led the system to deliver to this char two messages from two players with their account numbers and passwords!!! He realized that everytime he posted ae on trade channel, he would receive new accounts and passwords!
There is no explanation about why it happened. The owner of this char explained all of it here in a long livestream:
There are a few videos about him, but only in portuguese! You can ask to the former owner of Px in the link above if you wish, I really recommend because there is no content in english about him.
This guy can be potentially the sole cause of the famous "mass hacks" from 2 decades ago, where people got robbed several times without any explanation or reasoning at all.
Here is a full text from an official fansite article about the case (in portuguese too): https://portaltibia.com.br/o-curioso-caso-de-px-mais-uma-lenda-tibiana/
Px was a player from Eternia, and the whole case started in 2003. I wasn't a Tibia player back then, but I wonder if people outside the brazilian community knows anything about that. The player used to steal account in Cyber Cafes, with crowds of people behind him and also made a lot of money selling items from Tibia.
Nowadays, the character is long gone due to a Tibia investigation that started after sometime that the whole incidents reached the peak of hackings. Px apparently hacked more than thousands of accounts according to him! He did this without any fear or remorse and was only stopped much later due to an investigation led by Cipsoft directed to him.
I really can't even imagine how this can be credible or viable, but the guy got a lot of sources to sustain his claim. Here is an old image from his personal photo blog with his first hack (got from the article posted by PortalTibia):
(no link to the original photo post as far as I know)
Why I posted it since it seems so absurd to believe on it? Well, a good amount of people from Brazil knows this case and tells everyone that indeed we had such problem in the past. I am shocked due to the fact that nobody else talks about it nowadays. The youtuber claiming responsability showed a lot of evidence about the case.
EDIT: Found an english topic about the case from Tibia Royal: https://www.tibiaroyal.com/en/forum/topic/674/the-biggest-bug-of-all-time-massive-hacking-on-eternia-in-2004/
EDIT 2: The user Bobob from Tibia Royal gave an interesting explanation about it:
It happened, he wasn't the only one hacking accounts and it wasn't due to a bug in the game. What happened was people were using a new cheat program using state-of-the-art methods that had become popular, unbeknownst at the time however was the cheat had a hidden backdoor inside designed to steal their accounts by monitoring the chat for a certain message (ie: "Ae") then private messaging that person the account data. CIP themselves would go on to find out about this cheat's backdoor and subsequently used it as well to find & ban a large number of cheaters which they made a news post on at https://www.tibia.com/news/?subtopic=newsarchive&id=237.