Cels Forum

Sounds like a standard correction layer do to the wrong
paint being used on the first cel. As I understand it, they
will just copy the outline again onto another layer, 
repaint the area that was mispainted and then place it
over the first layer where it is then shot for cel image. 
Usually they wind up stuck to the original layer and one
may not realize a correction layer was used unless you
take the cel out and examine it. Several of my cels are 
like this and it usually seems to be the case that this 
occurs when the wrong shade of paint was used due to 
incorrect reading of what should be in shadow. My guess is
that the incorrect shade is close enough to the true that
a correction layer is possible without it being noticable
during shooting.

My 2 cents,
Marcus