This problem with a popup saying MathJax is inconfigured seems to never be backed with a reliable repro. Looks like I have one.
The problem is reproed in default browser availavle on Android 2.3.5 (device is HTC Desire S if that matters) when opening this question on Electronics SE.
The problem is only reproduceable on a mobile version of the site. It is not reproduceable every time - the chance is something like 70%, so it sometimes takes a reload or three to repro it. The popup can appear anywhere on the page, so one might need to scroll the page to see it. The default browser on Android doesn't show the page source, so no idea what hypertext is served by the server and no idea if it's wrong hypertext being served or some problem when actually rendering the page in the browser.
My guess is that the mobile version of the site is tested a bit worse that the usual one and at some point incorrect hypertext is served. Just a guess though.
This problem is super annoying and looks like there is a more or less reliable repro. Could you please try addressing it?