A Simon's Cat fan film is the one (and I just duplicated my experience).
I've got Flip4Mac installed.
Edit: By the way... Sorry for omitting this info from my original post.
All i get from viewing that viddy is a "ytimg.com" cache (and TOC.plist was left bare), so perhaps Flip4Mac is convincing Safari that a plug-in should be displayed as well. ::shrug::
Oops... I also had a ytimg.com cache item, but I had ⌥--E'd it away, so it slipped my mind.