When I have enabled "fast preview" in the MathJax context menu "Math Settings $\to$ Math Renderer $\to$ Fast Preview", MathJax will attempt to render formulas in the "side-by-side markdown" comparison view for edit histories and suggested edits. This view is otherwise supposed to show raw unrendered source (and does so without fast preview enabled).
This error only shows when "side-by-side markdown" is the preferred mode when the page loads. It looks correct if I switch from "inline" to "side-by-side markdown" after the page load, but reappears if I then reload the page.
Here's how a random example looks for me (Firefox 35.0.5, Windows 7) with Fast Preview enabled: Note that the red/green highlighting seems to confuse MathJax into losing sync of the beginning/ending dollar signs.
This behavior doesn't show in Chrome 39 or IE 11.