This link works fine when it's copy/pasted:
_ gets encoded as
%5f when it's autolinked, and this can break the URL:
Edit: Admittedly, the issue is related to GitHub in this case:
$ curl -I "https://github.com/cyga/www%5ffdw/" HTTP/1.1 404 Not Found Server: GitHub.com Date: Mon, 14 Apr 2014 13:42:08 GMT Content-Type: text/html; charset=utf-8 Status: 404 Not Found Cache-Control: no-cache X-XSS-Protection: 1; mode=block X-Frame-Options: deny Content-Length: 226672 X-GitHub-Request-Id: 5062C074:7D81:1D8C557:534BE5AF Strict-Transport-Security: max-age=31536000 X-Content-Type-Options: nosniff
But Stack Overflow could work around the problem by not encoding the
_ character to begin with.