Comment 2 for bug 641809

Well, some sites check referer and respond with 404 or 403 or whatever if referer doesn't match some pattern. This way they prevent, say, hotlinking.
Like this one - http://www.gamersyde.com/download_deus_ex_human_revolution_tgs_trailer-17226_en.html. It renders "private" URL to actual downloads but just pasting this URL to Steadyflow results in 4kb (or something like) HTML instead of ~70M video.