Tmobile.com is having some serious problems

T-Mobile's site has been having some major issues for several days now, bad enough to make the site unusable. 400 bad requests everywhere (among other things). Even the salespeople in their brick & mortar stores can't order anything because their systems are only working intermittently. They are apparently in the middle of rolling out a new site design - some pages are in their new modern looking theme and some are not. As some one who is primarily a desktop and embedded developer, I can only wonder what causes this kind of thing. I admit it, the only sites I have been responsible for were small and simple and just worked. But I'd be interested in reading about some of the things that can go wrong in a more major rollout. Anyone have any interesting stories, or speculation about what went wrong in this case?