I'm used to entering the URL https://<host>/wiki/Special:UserLogin to login with a different username. It would warn "You are already logged in as S Page (WMF). Use the form below to log in as another user." and let me do just that.
However, now when I do this on beta labs when I'm already logged in, I get a 302 redirect and then Internal error exception from http://en.wikipedia.beta.wmflabs.org/wiki/Special:CentralLogin/complete?token=921d7ed90f5b780db73c89673f1bb6d6 "The user account logged into does not exist." The full display is below.
The workaround is to click Log out or visit Special:Logout, then log in.
As I recall, visiting Special:UserLogin while logged in on mediawiki.org a week or so ago would just redirect me to the main page (not ideal but better than an exception); but now I'm getting the same Internal error exception except the production cluster truncates it to a pink "[7ed77f38] 2014-08-21 08:02:15: Fatal exception of type MWException".
Below is the full error display on beta labs:
[82d43ccf] /wiki/Special:CentralLogin/complete?token=921d7ed90f5b780db73c89673f1bb6d6 Exception from line 167 of /srv/common-local/php-master/extensions/CentralAuth/specials/SpecialCentralLogin.php: The user account logged into does not exist.
#0 /srv/common-local/php-master/extensions/CentralAuth/specials/SpecialCentralLogin.php(33): SpecialCentralLogin->doLoginComplete(string)
#1 /srv/common-local/php-master/includes/specialpage/SpecialPage.php(363): SpecialCentralLogin->execute(string)
#2 /srv/common-local/php-master/includes/specialpage/SpecialPageFactory.php(516): SpecialPage->run(string)
#3 /srv/common-local/php-master/includes/MediaWiki.php(300): SpecialPageFactory::executePath(Title, RequestContext)
#4 /srv/common-local/php-master/includes/MediaWiki.php(609): MediaWiki->performRequest()
#5 /srv/common-local/php-master/includes/MediaWiki.php(460): MediaWiki->main()
#6 /srv/common-local/php-master/index.php(46): MediaWiki->run()
#7 /srv/common-local/w/index.php(3): include(string)