Most MMOs have hundreds if not thousands of quests, textures, user-interface elements, statistics, NPCs, zones, instances, skills, and animations. On top of that they try to provide an environment where thousands of players can interact with each other without causing noticeable client or server performance issues. Obviously, there are many complex pieces to an MMO and messing up something small can have huge unforeseen consequences.
Ironically, patches are intended to fix game issues but for some reason they often break something else in the process. No developer is immune to this phenomenon.