Since cels are painted by humans sometimes
mistakes are made.  Sometimes the mistakes
are caught and a correction layer added and
sometimes the painting mistake makes it into
the production.

Another example is a nice Xellos cel that Ryen 
has in her gallery, 
http://ryen.net/cels/slayers/33.jpg
Xellos's shirt sleeve is miscolored black 
instead of yellow.  

You would have to know the episode image came from 
and watch to see how cel appeared to know whether
there is a correction layer missing or cel was 
filmed that way.