When I miss getting a cel I could have gotten 
(typically because I think about it too long, or 
I forget to bid before an auction ends and 
remember too late) it haunts me for a long time.

I even have a folder (named "Ones that Got Away") 
on my PC, containing images of these cels.  I 
periodically torture myself and tell myself what 
I fool I was by looking at these images.

On the other hand, it does act as a kind 
of "shopping list" or want-list.  I never make 
the same mistake twice.....