- Changes. Right up to launch, fixes are being made and tested, but these improvements are not being tested as long as the changes that were made several weeks earlier. What seems to work at Blizzard QA and during the end of the beta, may fail miserably when a jillion people use it on launch day.
- Not all bugs that were found were reported. Maybe Blizzard's QA is far more talented than most, and no one is lazy and/or disgruntled, but most testing departments have a bad apple or 6 that can't be bothered to enter in all the bugs they find. Also, beta testers often think "Well, that must have already been reported," and don't do anything about it. Or "I'll write it up after ... oooh shiny!" If the programmers don't know about the bugs, amazingly enough, they don't get fixed. Hush, Captain Obvious.
- All the the bugs cannot be found. Really, at no time can any developer say that a game is bug-free. Even if all the developers are 100% efficient and all the testers are perfectly diligent, a player in live is still going to accidentally fall through the world when trying to cheese a shortcut to a quest objective.
- Some bugs are deferred. One of the biggest things I had to get over as a tester was the fact that many bugs are known about but considered acceptable until the next patch. The producers (or however it is done at Blizzard) have to decide which bugs must be fixed (an easily reproduced crash) and which can wait for a patch or three (a text bug). Sometimes a crash that is difficult to repeat will be deferred and then it will turn out that players can repeat it just fine.
It's open warfare between Alliance and Horde in Mists of Pandaria, World of Warcraft's next expansion. Jump into five new levels with new talents and class mechanics, try the new monk class, and create a pandaren character to ally with either Horde or Alliance. Look for expansion basics in our Mists FAQ, or dig into our spring press event coverage for more details!