The sweet, passionate melody captivated his heart from the first note; it was full of radiance, full of the tender throbbing of inspiration and happiness and beauty, continually growing and melting away; it rumoured of everything on earth that is dear and secret and sacred to mankind; it breathed of immortal sadness and it departed from the earth to die in the heavens - Ivan Turgenev
I think youtube only wants to support embedding based on the URL construct defined in the "Share" tab, so grab the URL from there, THEN remove the "s" and presto... embedded video.
If we could get a Mac enabled weenie to reproduce the error and fix, that would be great. Here's a little screenshot splaining where to grab the correct (Share tab) and incorrect (Address bar) URLs from.