What's wrong with the software today? When I'm typing or editing an answer, the $\TeX$ doesn't get rendered and all sorts of strange messages are splattered across the screen. After I've saved the answer, I see a similar thing until I hit the "reload" button.
I solved the mystery!
There is a deleted answer which caused this. This is why users with reputation below 10,000 did not have the bug anymore. I have edited this answer to remove the "malicious" code. All should be fixed.
I can confirm the bug report, I got this on this answer. Refreshing didn't help, nor did a hard refresh (Ctrl+F5).
The screenshot from a minute ago or so.
Firefox 16.0.1; Arch Linux (kernel 3.5.6-1 x86-64).