While it's not that common, it happens more than 
I'd have expected. They're different from this 
fancel which is a fraud. 

The non-prod cels are usually created by the 
studio, but were rejects or not used for other 
reasons. 

I even have one that was purchased from a brick 
and mortar cel shop while I was visiting Osaka. 
Oddly enough, I found another non-prod one online 
of that exact same cel. They're shown in the TM 
section of my gallery.  

- Chi