You've probably came across this message on YouTube, right?
Or maybe that other message saying that the current video has restricted content and you have to login and verify your age to see the movie, right?
Well, there's a way to bypass that restriction. YouTube video links have the following format:
where VIDEOID is that sequence of letters and numbers that univocally identifies a youtube video in their huge database. The access to the video through this link format will feature any restrictions that the uploader may have set.
However, a simple hack lets you see the movie (but only that, since this link will only give you access to the YouTube embedded player). Just change the link to the following format:
And that's it! Now you should have access to the same video without any restrictions.
But I guess this won't work for long. As soon as YouTube figures out that this "back door is opened", they'll probably apply the same restrictions and "close" it for good.