MathJax loops infinitely when \mathchoice is used in an munderover.

The \mathchoice macro needs to know the displaystyle and scriptlevel before it can choose which of the four alternatives it needs, but munderover needs to know its children's core mo element (if any) in order to determine its child's scriptlevel. That leads to an infinite loop when \mathchoice is used in the under or over position of munderover.

See this user form post for the original discussion.