Currently, sphinx-math-dollar can encounter pending_xref_condition-nodes which will raise an exception. (see sphinx-doc/sphinx#11121).

An example repository that shows the bug is found here: https://github.com/randolf-scholz/sphinx-demo

In short, since sphinx does not officially want to register pending_xref_condition as a node (it is considered an implementation detail), unknwon_visit needs to added to avoid raising the NotImplementedError from the base class.