Go to a page like https://en.wikipedia.org/wiki/Wikipedia:Sandbox?action=info. The following message appears, without any skin interface:
A database query error has occurred. This may indicate a bug in the software. Function: InfoAction::pageCounts Error: 2013 Lost connection to MySQL server during query (10.64.48.27)
Presumably what this really means is that some sort of internal timeout has occurred.
Ideally the error should be reported in a more friendly fashion. It is not, after all, a "bug in the software" that the error occurs; it's just a DB timeout.
Or, even better, handle these internal timeouts more cleanly and just present the action=info page without the unavailable information.