On sites where MathJax is enabled, when the "show markdown source diffs side by side" view is used in the revision list (this thingie: 
), MathJax will attempt to interpret content surrounded by $ signs in the markdown source. MathJax should be disabled for the markdown source blocks.
On the screenshot below this is visible as framed text (this is what MathJax does when it cannot interpret something). This is a rather minor issue, but it's still incorrect behaviour.
Here is an example with a screenshot:
https://mathematica.stackexchange.com/posts/1513/revisions
Note: @Matt pointed out in a comment below that "this will not happen if you have another viewing mode enabled, and then switch to markdown side by side. You have to have markdown side by side enabled by default (basically select it then F5)"
