I came across a Wikipedia page that had imbedded errors like “Failed to parse (Cannot write to or create math output directory)” in all the places where there was embedded math. Turns out MediaWiki uses TeX to render math stuff, and caches the output. It appears that the particular server in the Wikipedia server cluster that was supposed to render the math for that page was hosed somehow, and a cached version was generated with the math errors.
Luckily, it’s easy to get Wikipedia to purge a cached page. I chose the first way: just append “?action=purge” to the hosed page’s URL. It worked like a charm. A quick Google search shows that this happens from time to time, since Google has cached Wikipedia pages in the hosed state. I checked the other Wikipedia pages that Google showed with that error, and found only one still in that state. So I purged it. Wikipedia janitorial services, I guess.