It seems Oddthinking managed to break the automated linkifier while linking to a docx file. I'm not positive it has anything to do with .docx files, or perhaps urls without a
www. prefix, but the effect can be found in this sandbox answer.
Here's the question it occurred on (now fixed, although I'm not sure what i did)
Edit: It wasn't related to the .docx extension at all, there are weird, non-printable chars in the URL. Could these be stripped out perhaps?