You could have seen the videos on YouTube are restricted in you country. In this case YouTube displays messages as follows:
"This video is not available in your country." or
"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."
The trick to get them watched is changing URL a little bit as following; e.g. if you get a restricted video with URL like this:
You just need to replace it to this:
As you're seeing, it's simple to understand the changes, so what you only need to change is to take away "watch?" and replace a "slash" with "=" as above.
With this simple changing you will launch "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."(Google OS)
Read more and see the screenshots on Google OS.