I was noticing that it is not uncommon for YouTube videos to become 'unavailable' for any number of reasons (usually due to a claim of copyright infringement). It is also not uncommon for the same clip to be available either again or elsewhere on YouTube.
If you notice that has happened, and you posted it, there is an "Edit" button you can click and go in and replace the old information with new. If you are unable to find a new link, you also have the option of choosing to delete the post.
If you come upon one that you have not posted, please let me or one of the other moderators know, and we'll see what we can do to fix it.
I've gone through and fixed a few, but it is a little time consuming. I will do a few at a time, as I am able.