Update: I later found that anything I edited on Main didn't render properly in Preview. On a whim, I de-activated the LocalCDN plugin I installed after seeing a-super-easy-way-to-speedup-mathjax-loading-with-localcdn-add-on on Meta. Everything is now fine and my edits are previewing with alacrity.
I was editing am301's answer on this question and found that the Mathjax didn't render properly in the preview window. I have since copied the answer into a new question from where this screen shot is taken:
You can see that the first sum is rendered correctly but the second formula is not, nor are the subsequent ones. The peculiar thing is that if I copy it into the sandbox on Meta, it all renders perfectly. I'm using Firefox on Windows 7 and I've tried different Mathjax renderers.
What is causing the Mathjax to be rendered incorrectly, and why is it different on Meta?