I just noticed that this diff (select "side-by-side markdown" mode if it's not already selected) is showing MathJax where it doesn't belong:
(MathJax right-click menu included to show that those italics are indeed MathJax.)
Chrome's DOM inspector shows that the diff in fact does have a parent element with the class name
markdown-diff tex2jax_ignore, which should cause MathJax to ignore its contents. Clearly, however, this is broken somehow. I suspect either that some JS code is calling
MathJax.Hub.Typeset directly on one of the inner elements, or else that the MathJax config specifying the class names to ignore is somehow broken (which, at a glance, it doesn't seem to be).
Either way, this is clearly a bug, and should be fixed.
Ps. This seems to be a regression of When "show markdown source diffs side by side" option is used, MathJax will try to interpret $ signs in markdown source and Broken LaTeX rendering of markdown differences, both of which have been marked as fixed back in 2012.