Error 404

[mdmcginn]

michael@doulos:~$ youtube-dl --verbose "http://www.youtube.com/watch?v=_yLZuN5QUYU&list=WL6dkgH7wD33qPAgtPL6E9umX3ZXckvNF4"
[debug] System config: []
[debug] User config: []
[debug] Command-line args: ['--verbose', 'http://www.youtube.com/watch?v=_yLZuN5QUYU&list=WL6dkgH7wD33qPAgtPL6E9umX3ZXckvNF4']
[debug] youtube-dl version 2013.07.19
[debug] Python version 2.7.4 - Linux-3.8.0-26-generic-i686-with-Ubuntu-13.04-raring
[debug] Proxy map: {}
[youtube:playlist] WL6dkgH7wD33qPAgtPL6E9umX3ZXckvNF4: Downloading page #1
ERROR: Unable to download webpage: HTTP Error 403: Forbidden; please report this issue on https://yt-dl.org/bug . Be sure to call youtube-dl with the --verbose flag and include its complete output.
File "/usr/local/lib/python2.7/dist-packages/youtube_dl/extractor/common.py", line 121, in _request_webpage
return compat_urllib_request.urlopen(url_or_request)
File "/usr/lib/python2.7/urllib2.py", line 127, in urlopen
return _opener.open(url, data, timeout)
File "/usr/lib/python2.7/urllib2.py", line 410, in open
response = meth(req, response)
File "/usr/lib/python2.7/urllib2.py", line 523, in http_response
'http', request, response, code, msg, hdrs)
File "/usr/lib/python2.7/urllib2.py", line 448, in error
return self._call_chain(_args)
File "/usr/lib/python2.7/urllib2.py", line 382, in _call_chain
result = func(_args)
File "/usr/lib/python2.7/urllib2.py", line 531, in http_error_default
raise HTTPError(req.get_full_url(), code, msg, hdrs, fp)

[jaimeMF]

The problem is that it interprets the url as a playlist (#755), I think it's from your "watch later" list, which is private (#1073).
It will work if you do youtube-dl "http://www.youtube.com/watch?v=_yLZuN5QUYU".
Thanks for the report.