I'm seeing [Math Processing Error] all over this site today. Is there a bug or is this problem unique to me? How do I fix it? Thanks.
Per popular demand:
One of the first things you should do when MathJax is crapping out on you is to try a hard refresh, which is executed by holding your Shift key and then clicking the Refresh/Reload/whatever button in your favorite browser. Browsers usually cache stuff like images and JS bits to ease the task of loading a page. Since this cache is sometimes prone to getting crap on it one would sometimes want to do a hard refresh so that the browser is forced to pull stuff from the servers instead of from its cache.
The more drastic method is to do an actual cache clear (see your browser's help file for details) and restart. If all else fails, consider dropping a line here on meta, or for really severe problems, the MathJax team.