A good part of my collection (at least until the new ones 
come in!) are SM, and I've heard that its VERY common for 
Toei (sp?) cel lines to fade.  A lot of mine have only faint
brown lines where there should be black.  And, as someone 
else pointed out, if you collect SM you've seen a LOT of 
cels where Sailor Moon's bow is icky brown.  

I just make sure I keep all my cels in closed cel books in a
room thats almost always dark - except when I have to get 
them out to drool over!

Az