For a long time one of my favorite cels was the 
top one in the image attached: Sakura using The 
Sword in the episode in which she visits Tomoyo 
at her house and captures Shield.  I noticed that 
the colors in the broadcast version were 
different, but I figured that this was due to the 
way in which the cel was lit when it was 
photographed.

Then the cel below came up for auction on YHJ.  
The colors in that one match the broadcast 
version.  And it is labeled A34, while the top 
one in "normal" color is labeled A3. But 
otherwise both seem to be genuine studio products.
(I put both up in my RS gallery, and you can 
click the link above for more detail.)

I'm guessing the top one was a test cel.  But 
does anyone have any further insight about why 
these two identical but different-colored cels 
should exist?