I *think* I've seen what you are talking about.  I
have seen something that looks like the paint and
the black line are warping into each other.  (I've
had the best views in person of course ^_^;)  I've
only seen it in relatively old cels.  So, whenever
I see an old cel in person on online, I try to see
if I can see evidence of this effect.  I suppose
like line fading it might just be another sign of
age and inconsistent care.  

Keys