Occasionally, I'll want to look through my older questions or answers from my profile page. There have been a few instances (including just a moment ago) when clicking on the next page in my list of questions will cause the page to render without CSS - a right ugly mess.
I believe this also happens when I change the sort order (votes, views, etc).
This is the only page I've ever seen lose it's CSS on SO.
Edit: It just happened again when I tried to order my questions by views. The page now shows only the questions section, with no CSS. The address shown has also changed to "http://stackoverflow.com/api/userquestions.html?userid=1588&page=1&pagesize=10&sort=views". I still have the tab open, and have Firebug installed. I've uploaded a cropped image of the tab.