Heading to https://meta.wikimedia.org/wiki/Special:GlobalUserRights and submitting the form without filling a username cause:
#0 /srv/mediawiki/php-1.32.0-wmf.20/extensions/CentralAuth/includes/specials/SpecialGlobalGroupMembership.php(102): MWExceptionHandler::handleError(integer, string, string, integer, array, array)
#1 /srv/mediawiki/php-1.32.0-wmf.20/includes/specials/SpecialUserrights.php(469): SpecialGlobalGroupMembership->fetchUser(string, boolean)
#2 /srv/mediawiki/php-1.32.0-wmf.20/includes/specials/SpecialUserrights.php(199): UserrightsPage->editUserGroupsForm(string)
#3 /srv/mediawiki/php-1.32.0-wmf.20/includes/specialpage/SpecialPage.php(569): UserrightsPage->execute(string)
#4 /srv/mediawiki/php-1.32.0-wmf.20/includes/specialpage/SpecialPageFactory.php(581): SpecialPage->run(string)
#5 /srv/mediawiki/php-1.32.0-wmf.20/includes/MediaWiki.php(288): MediaWiki\Special\SpecialPageFactory->executePath(Title, RequestContext)
#6 /srv/mediawiki/php-1.32.0-wmf.20/includes/MediaWiki.php(868): MediaWiki->performRequest()
#7 /srv/mediawiki/php-1.32.0-wmf.20/includes/MediaWiki.php(525): MediaWiki->main()
#8 /srv/mediawiki/php-1.32.0-wmf.20/index.php(42): MediaWiki->run()
#9 /srv/mediawiki/w/index.php(3): include(string)
#10 {main}101 function fetchUser( $username, $writing = true ) { 102 if ( $username[0] == '#' ) { // <----------------------------------------------- oops 103 $id = intval( substr( $username, 1 ) ); 104 $user = CentralAuthGroupMembershipProxy::newFromId( $id ); 105 106 if ( !$user ) { 107 return Status::newFatal( 'noname', $id ); 108 } 109 } else { 110 $user = CentralAuthGroupMembershipProxy::newFromName( $username ); 111 112 if ( !$user ) { 113 return Status::newFatal( 'nosuchusershort', $username ); 114 } 115 } 116 117 return Status::newGood( $user ); 118 }
From Special:GlobalUserRights