Mismatched XML causes mysterious-sounding errors

(Bug, To be Fixed , Priority: Medium, Test Status: No automated tests yet , Reported By Justin du Coeur, )
QText
Summary: Because we sometimes do XML parsing of the Wikitext, mismatched HTML tags can cause loud and hard-to-understand failures. For example, put a simple <br> tag (not XHTML-styled) into an _edit control to see this happen.
We probably can't prevent this, but we should at least produce a more useful error that calls out mismatched tags as a possible source, and document this carefully.