Every now and then YouTube deletes a video, normally because of copyright issues. But, there is a way to still watch the deleted videos which is quite clever.
Deltube have put together a script which will allow you to watch deleted videos. All you have to do is enter the ID of the YouTube video that has been deleted by YouTube, and it will magically start playing. Most recent videos will work, but you may have problems with older videos as YouTube eventually physically deletes videos.