I don't think that an incorrect episode number 
would devalue the layout. Just going from my own 
experience, I knew about the incorrect numbers on 
the two I have *before* I bought them and I still 
bought them. ^_^

I had also asked here why the numbers might be 
wrong, and most people's thought seemed to be the 
scene was switched to a different episode last 
minute and the number was simply never corrected. 
But in my case, I had already found the scene in 
question (just in a different episode). In your 
case, since you haven't found the scene yet, it 
is possible your layout is from a scene that was 
cut or changed. It's not unheard of -- even in 
cels. I have seen collectors with cels from 
Escaflowne scenes which were cut, and I owned a 
Psychic Force cel that didn't match up to any 
scene in the OVAs.

If nothing else, art from a cut scene is a 
rarity. ^_^

Many Sharp Smiles,
--Drac