The css data from MediaWiki:Group-sysop.css is intermittently not loading. This is causing some issues for sysops, as some elements that are only useful for sysops (but not 'harmful' for everyone else) are often hidden in common.css, with an override in the corresponding group-xxx.css files to !important-ly display the elements again.
Examples are in:
With a sample test on testwiki, the rendered page says it knows the account is a sysop (in ,"wgUserGroups":["sysop","*","user","autoconfirmed"] ) however, the group-specific stylesheet is not linked to the document code.
See also discussion at: https://en.wikipedia.org/w/index.php?title=User_talk:Xaosflux&oldid=909681787#What's_broken?