In the time between about 9:25 and 9:50 (-0200) today, I observed in the ksh Wikipedia that the local version of [[MediaWiki:Sidebar]] was ignored in all pages *), and the default one from the (fallback) MessagesEn.php was being used in the monobook skin. First assuming that, maybe, someone else had altered, or deleted, the page [[MediaWiki:Sidebar]], I found its content intact and unchanged, yet not being shown in the left edge of the page.
Next, I tried the same page with &uselang= which changed the language but not the sidebar layout (which is as it should be), still using the fallback default version of the sidebar.
Next, I tried [[MediaWiki:Sidebar]] with &action=purge, and got the local sidebar at once.
From that moment on, the site behaved as usual.
I have no idea how the wrong Sidebar could sneak into the cache but, apparently, it did.

MessagesKsh.php had been updated in svn roughly 4 days ago. I do not know, when the update became effecitve on the WMF servers, and whether or not this may have affected the page cache of kshwiki. At least, of course, $messages['sidebar'] had not beeen altered with the update, see: http://svn.wikimedia.org/viewvc/mediawiki/trunk/phase3/languages/messages/MessagesKsh.php?r1=34273&r2=34598

*) As far as I can tell. From the moment, I first consciously looked at it, all pages that I tried were affected.

Version: 1.13.x
Severity: enhancement
URL: http://ksh.wikipedia.org/wiki/MediaWiki:Sidebar