Maintenances?

[Raeletta]

There's always bugs that won't be uncovered until they're released at a huge scale. Yeah, they test them out as much as they can before launching it, but they can't find every single possible situation or have thousands and thousands of people doing things at once like the playerbase will be.

This.

Always expect one. That said, on the whole, there are so very few bugs with each patch that it's hardly a problem. If the patches were lower quality you'd see more maintenances, maybe at less convenient times. Expecting perfection however, is just unrealistic.