Interesting post from WoW general forum.

[Malakar]

... Just writing some code can cause problems with the core when you are not even trying to effect the core.  Bugs occur in ways that don't make sense because a bug is a logic error, and humans are not 100% logical when they code.  As such, when trying to add more value to a program, it is quite possible to introduce a new bug that effects the core program when you don't even try to effect the core program, that is the way things are.

You're quite right about that. However with good coding techniques you can keep this to a minimum, and after some debugging time, most of these vulnerabilities between separate components can be dealt with. Perhaps this problem could be exacerbated a lot depending on how the team of programmers work together though. And yes I realize that this problem is naturally exacerbated to some extent by a team being required in the first place.

Also, I'd expect that at some point, content updates in WOW will be mostly data and little code. New instances, monsters, spells, etc may require code sometimes, but probably not as often.