Jul 24 2017
Dec 29 2016
Jul 25 2016
Happened on Special:Undelete as well when trying to view a deleted article.
Jul 21 2016
Happened to me a few minutes ago when attempting to block a user at the same time as another administrator completed the action. Page should have been a block conflict one.
Sep 30 2015
Happened to me on Simple Wikipedia and Mediawiki.org