Originally Posted by
Aliessil
After 15yrs of working in Software Development I can assure you there's virtually no bug-free software anywhere on the planet. This isn't likely to change anytime soon without a major technology shift. The bigger, the more complex a piece of software is, the more bugs it's likely to contain, and I'm betting WoW runs to tens of millions of lines of code by now.
That small, simple bug you've found? It could be caused by a single line of code, or it could be from the interaction of dozens or even hundreds of lines in entirely unrelated areas. It's not uncommon to find that it's easy to fix that one bug but in doing so, you create one or more others, and maybe they're worse. Then there's the bugs that only become visible when you use specific hardware - maybe you've upgraded your system, or maybe Blizzard have upgraded to more powerful servers with the latest Intel chip, and it's that chip causing the problems.
Maybe it happens at some weird combination of events - it has to be within two hours of noon, you have to be on Pandaria, there needs to be a mage casting fireball within 100 yards and there must be a virmen within vision range. I bet that specific scenario happened a lot at the start of the expansion, but good luck finding that bug!
Software testing is a balancing act between minimising the effect of any bugs that get through, and getting your software out in a timely manner. If you want e.g. Warlords of Draenor to be released 100% bug-free, be prepared to wait years, and don't upgrade anything in the meantime.
As I said, this isn't going to change soon without a major shift in technology, and I've no idea what that will be.