While loading https://www.mediawiki.org/wiki/Special:Code/MediaWiki/91123 I found that some sort of "mobile user survey" banner displayed in the sitenotice area, and a mixed-content warning popped up in my browser indicating something was loaded over unencrypted HTTP.
I popped open Firebug to check the network panel to find the culprit and reloaded to populate it, but the banner disappeared and hasn't reappeared so can't confirm it just yet.
Version: unspecified
Severity: major