I RE'd WoW for ~10 years. You'd be horrified if you knew how many hacks and crazy-bad technical designs are in the game. The team sometimes infamously has (or had, as of a couple expansions ago) hundreds of thousands of open issues and sometimes squashes bugs which are several expansions old.

Incidentally, they also quite often create new bugs in super-old content due to changes in newer content.

I mean, the work they do is incredible. But it blows my mind how they manage to maintain this game given the amount of intertwined legacy within it. I'm not sure what exactly caused it to become as bad as it did. It's probably a mix of the game being far more popular than they originally anticipated, the team being incredibly large, and the engine itself not being very accomodating.