I'm not able to find the most generic version of this bug, so this is only a singular example of it:
<math>K^\operatorname{op}</math>
renders correctly as PNG, but MathJax only shows an error. Instead you have to write
<math> K^{\operatorname{op}}</math>
Perhaps related to bug 35186 comment 6.
Version: unspecified
Severity: normal