The first time I accessed the TeX examples page (/demos/tex-samples/) they showed up, after a few seconds of displaying the raw TeX, as a red message reading [Maths Error] all over the page.
After going back to the demo index page, viewing the other example pages (which all worked) and then returning to the TeX examples, it showed formatted equations that looked okay.
I am using WebKit r64727.
Can you open the contextual menu and select "About MathJax" and tell me what font mode is listed (just under the MathJax heading)?
I have seen some similar behaviour intermittently when image-font mode is used, but am not able to reproduce it reliably. I'll see what I can do. If you have a way of forcing it to occur, let me know.
Davide