When you hire QA testers for software, you expect them to do everything they or the software engineers could think of to break things, then end up realizing that the general public may well come up with things that none of the QA testers or software engineers thought of. Even an open beta isn't going to find everything. You'll have some people in the open beta actively trying to break things, and likely duplicating what the QA testers already tried. You'll have others who just use the software normally without trying to break anything. They'll only find bugs which are obvious and easily triggered via normal usage. There may be a few who end up doing weird things. Things that make you wonder "why would anyone think to do that?"
And then in MMOs, there are bugs which only crop up one live servers because the bug is caused by a combination of server load and bugs in the code. Or because with a much larger number of people interacting with the code, there's more people doing bizarre things which cause the bugs to be discovered.
For a non-MMO example, Skyrim is well known for being a buggy mess with easy to trigger exploits such as item duplication and infinitely stacking enchanting/alchemy buff loops. Yet here's the thing, in the course of normal game play none of those easy to trigger exploits are likely to crop up unless you know about them, and actively try to use them. You can get Stealth to 100 in the tutorial via exploiting a specific NPC and mechanics. But that'll never happen unless you go out of your way to do so.
EDIT:
It always feels weird to me when I end up preaching patience and recognizing that MMO updates come bundled with bugs being a fact of life. I mean, I use to be one of the people who'd post angrily about down time and bugs.
