The development has to service at least 3 versions of the game at once. The current live version might need emergency patches (exploits, crashes). The current patch you are working on is another and is very near the live version. And there is the future large patch you are working on, new features and expansions. Can you imagine how complex it is to merge those versions to make a stable patch? Its tough and to be honest if there wasn't Perforce I wouldn't know how this can be done.
But developing a new version is one side. The other side is worse: Testing is a nightmare. A MMO is a complex game, so if you change fundamental things how do you test the beast? It's nearly impossible to test all sides of a live MMO and it gets worse the older the game gets: complexity is added nearly every month.
So it is natural that bugs slip by, some hidden ones and some obvious ones where you ask yourself: why did they overlook this one?
A blog has some aspects listed of the recent Warhamemr Online Patch 1.01, and if you don't know this blog check it out, its pretty nifty: Keen & Graev's Gaming Blog