One day this cel appeared on YHJ:

http://sensei.rubberslug.com/gallery/in ... mID=366046
It was sold inside a cardboard frame identifying it as from the Unico pilot TV episode, produced in 1979 by Sanrio Studios, so that wasn't a mystery. But the guy in the upper left corner was a mystery -- he never appears in the pilot nor in any of the other Sanrio Unico productions.
But I'd gotten the first English translation of Osamu Tezuka's Unico manga a few months before, so at a glance I knew exactly what scene this scene was supposed to show. Unico gets into the fix he's in (having to lose his memory and shift to another time and place at the end of each episode) because of a feud between his original owner, the Greek mortal Psyche, and Lady Venus, who's jealous of her beauty. So she sends her son Eros to earth with orders to make her fall in love with the most disgusting male mortal he can imagine.
But instead (in the manga as in the original Greek myth) Eros bungles the job and falls head-over-heels in love with Psyche himself. That's an important scene in the manga, and Sanrio evidently storyboarded and animated this scene for the pilot, then cut it out as taking up too much time.
This cel of Eros survived because it was mated with a non-matching cel from another scene. (She looks like Psyche, but she's the evil duplicate created by Lady Venus to abduct Unico.) Still, it's a nice imagining of what this "accidentally falling in love" lost scene would have looked like.
Anyone else happen on something in an out-of-the-way place or auction that you somehow immediately recognized as something you knew?