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Example of video blocked outside US (Madonna - 4 Minutes): http://www.youtube.com/watch?v=I9ciR9qR1dU
To see it, paste this in the address bar: http://www.youtube.com/v/I9ciR9qR1dU
The same trick works if you don't want to log in when you get this message: "This video or group may contain content that is inappropriate for some users, as flagged by YouTube's user community. To view this video or group, please verify you are 18 or older by logging in or signing up."
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http://www.youtube.com/v/VIDEOID sends you to the player used by YouTube when you embed the video into a web page and this player doesn't perform country verifications and can't detect if you're logged in.