Go to the MathJax home page and change setting from HTML-CSS to MathML and then back to HTML-CSS. This results in the positioning of individual characters being wrong. (Firefox 3.6.10 on Windows.)
Oddly, do it again and all is well. And if done again one more time.
OK, I've confirmed the behavior and will look into it. No idea why that would be the case.
Davide