If it's already saved, it'll do a new snapshot, but I believe the archive is smart enough not to duplicate saved youtube videos.
I tried this and it keeps saying job failed.
Same issue. Love the concept of saving through a GET link.
Looks like the Archive is having some trouble today, but it's also getting throttled because the link is getting clicked a lot. the /save/ link will redirect you to a saved snapshot of the page; it basically tells the archive "capture this page right now".