The following comment (edited down from a real-world example) "
$\cong$—spacer *italics* spacer—$\mapsto$" should render roughly as "≅—spacer italics spacer—↦", but, on MathJax-enabled sites like MathOverflow, the stars render literally, rather than entering italics. This appears only to happen in comments, not in post bodies.
EDIT: See further exploration in a comment by @j.c., which I reproduce here as source:
Here's a slightly more minimal example: $\cong— *italics* \cong$: code `$\cong— *italics* $\cong$. The lack of a space between the MathJax and em-dash is necessary ($\cong$ — *italics* $\cong$: code `$\cong$ — *italics* $\cong$`), and there also must be MathJax on both sides ($\cong$— *italics*: code `$\cong$— *italics*`).