And from this and others I guess it's not an error, it's just something unusual.

I wonder if someone tampered with it for some reason and returned it to the cage.

The back has a little around the rim, the front has the coating, except for around the rim.

Originally I thought someone else may have found something like this, but I guess not.

Yes, it's staying in the collection, just because it's unusual and fun.

Now I better get back to work!