When I try to close a question as duplicate, it fails at the point where I am to enter the URL of the duplicate. The look up of that URL fails with the message “An error occurred while searching; please try again”. My browser's console shows a request to https://meta.stackoverflow.com/posts/popup/close/search-originals/96669 that's redirected to https://meta.stackoverflow.com/error?aspxerrorpath=/posts/popup/close/search-originals/96669 which returns a 500 status.
This doesn't happen on other sites. It's presumably a bug in rev 2013.9.27.1464. In case this is browser-related, I'm using Chrome 27.0.1453.93.