Hey, I saw a cel that had the 'correction' on it 
before.  It was a valgarv cel (slayers) and part 
of his hair was a different set of shades, so 
another cel was layered on top of it with hand 
painted corrections.  the person with the cel 
claimed it was more valuable (because of the 
rarity of hand-done corrections) but I don't know 
if retouching it would take away from the value.

If you want to sell it, probably not.  If you 
want to keep it, go for it.  -_-