Currently trending on the mediawiki-errors dashboard in Logstash. About 200 hits per 15 minutes.
> Math:
> Rendering failed. Printing error message.
Coming from regular page views for anonymous user cache misses and logged-in users alike.
At least one instance of it was caused by the presence of an EN DASH in the Math formula as part of a free text expression. It seems the parser fails when such a dash (–) is used in free text labels.
Example: [https://en.wikipedia.org/wiki/Testing_and_performance_of_IC_engines](https://en.wikipedia.org/w/index.php?title=Testing_and_performance_of_IC_engines&oldid=692339544#Air.E2.80.93fuel_ratio)
{F3221976}
>|_type | mediawiki
>|channel | Math
>|host | mw1240
>|http_method | GET
>|level | WARNING
>|message | Rendering failed. Printing error message.
>|referrer | https://www.google.com/
>|server | en.wikipedia.org
>|url | /wiki/Testing_and_performance_of_IC_engines
>|wiki| enwiki
Was fixed by making an edit and changing the EN DASH (U+2013) to a HYPHEN (U+002D).
Math should probably be fixed to not crash on seeing an En-dash. Alternatively, if this can't or shouldn't be supported, the message level should change from WARNING to DEBUG or INFO, since it is a user error in that case, not a software error.