Can't quite tell from the photo. Looks like about 90% of it is kind of goldish colored? Looks like a planchet problem at first glance. Can you get a better photo, maybe one that is zoomed out slightly? This one is slightly blurry.
If there is not an imprint of another coin on it, then there is no stamping issue. It looks to me like another nickel was placed on top of it and that other nickel had a chemical or adhesive or some other gunk on it, and it discolored the other coin. How does the back of this coin look?
Hmm. The issue is that if it was stamped over another coin, there would be some type of mirror image of the coin on your coin. Not sure at this point, but I do not think it is an error. It looks like whatever happened happened after the minting process.