Most of my surprises come when I get new lots and process them. I recall a cel that I got from a major CCS scene that I got at a steep discount because it was "stuck to sketch." When I got it, I couldn't help taking an old photo to explore where it was stuck and how badly. I gently probed ... and probed ... looking for the stuck spot ... and suddenly the sketch and cel slipped apart. No damage to cel or sketch, just a momentary dalliance.
I also get surprises as I learn more about what I have. One of the first sketch sets I got was
a set of four gengas from another key CCS scene. When I got it, I found that I had also gotten four more sketches, all done on light green paper. At the time, I had no idea what these were, but a more experienced collector on Animanga (anyone remember Animanga?

) explained that the light-green ones were the episode animation director's final corrections. Much later, I learned that these were done by CCS's character designer/chief animation director, Kumiko Takahashi.