I was on
https://bugzilla.wikimedia.org/show_bug.cgi?id=68521
and clicked in the link [[Special:UserLogin]] which appears in the bug title. This loaded a page like
https://en.wikipedia.org/wiki/Special:CentralLogin/complete?token=<some token>
which was a mostly blank page, containing just this:
[99330200] 2014-09-10 17:57:56: Fatal exception of type MWException
Version: unspecified
Severity: normal