I have some issue with the math font for quite some time now. And it only happens on this very site, and not on any other site.
Some of the letters, some of the time, don't render part of the top line of pixels. It's weird. If I increase or decrease the zoom factor in my browser, it goes away. But that's not a solution really.
Note the top of the $X$ and $Y$, as well the top of the $B$.
I have no idea what causes this, and this is the only site where it happens. It happens with Firefox, even in Private Browsing mode (so it's not an issue of cookies). With Chromium it works out just fine.
System details: Arch Linux 64bit, Firefox 26.0
The problem appears on Physics.SE as well, but not on Math.SE, as mentioned. This points out to a problem with the general layout (and in particular the font) of the sites (MO and Phy.SE use sans serif; MSE uses serif). I'm in contact with one of the MathJax developers, and further information will be posted when it is available.