I've seen it from the studio where they draw, 
with a felt-tip or whatever it is they use, right 
onto the top of the cel and not on a separate 
layer. I have several Tenchi cels like that. And 
I have also seen Sailor moon cels like that that 
were still in sealed bags. Its not terribly 
noticable unless you hold it at an angle. I would 
have to say that niether corrections to the top 
of a cel (done nicely of course) or line fading 
would stop me from getting a cel, but I wouldn't 
be willing to pay as much for it as one in good 
shape. A cel with lots of line fading could have 
been exposed to other damage (like the evil evil 
sun!) that might show up later in the acetate or 
paints.