Certain Youtube videos which are considered "inappropriate for some users, as flagged by YouTube's user community" are supposed to be hidden from users who are not logged in or who are not above 18 years old. But alas, google seems to have left a bug(or is it a hidden feature--not decipherbale by the underaged?) which allows you to watch any video you want without having to sign in or sign up.
Just give the direct path to the video like "http://www.youtube.com/v/video_id" and the video just opens up in the browser. This seems to have been left open to allow external websites to embed youtube videos. However, one would expect YT with the "brilliance" of google to have blocked embed access to the "marked" videos.
Apart from the flaw in youtube that has been hastily fixed by google, there seems to be minor bugs or rather design decisions that are confusing. Will they just keep patching things or consider a redesign of the model is interesting to watch for.