As Keys has said, a cel stapled to a background 
does not guarantee that the background is the 
matching, production one. I have an example of 
that from one of Violinist of Hameln cels.

http://www.ugcs.caltech.edu/~wpk/VoH1.html

Go to the Ocarina cel (second row, third cel). As 
you can see, the background that was stapled with 
the cel is similar to the actual one, but it does 
not match.

---Trebuchet