This issue happens on YouTube and related to formats 34 and 18. Format 34 is, for many videos, 360p widescreen, and of higher quality than the same video in format 18. However, sometimes format 18 provides a better quality version, as format 34 appears to be just 240p for some videos. There is no plan to fix this. Currently the only way of finding out which version gives higher quality is to download a chunk of both formats and peek into the files themselves to find out, if you know how. As, in my experience, format 34 usually wins, and in many future videos uploaded in high definition formats it will be 360p, format 34 will remain as the format to download in case of doubt.
+Due to changes in YouTube, youtube-dl is now forced to provide the final video URL directly instead of the ones it used to provide, which were based on the "get_video" resource. The new URLs are real and work too, but they need to be used in combination with the --cookies option to be useful for external applications. Issue 41 in the issue tracker contains a bit more information about the problem.
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