I have this keychain and not only do the backs look banged up ( on the one that CZ22 posted ) but, 2 of the candy charms look to be chipping the paint. If you look at the polka dot one and the orange/white checkered one, there are silver spots that look like the paint has completely chipped off.
Here are pics of mine for reference, it's never been used. It was bought at the outlet over a year ago.
When I first looked at the pics it seemed like all those marks might have been just an effect of the lighting, but I think you're right. I have the same keychain and used it as a purse charm maybe 5 or 6 times at the most and already the white one with the multicolor dots is showing a bit of the white enamel chipping off in exactly the same spot - what would be the lower right corner right under the smallest blue dot - as the one in cz22's photos, so it has to be from the charm just below it rubbing against it. The keychain in the first set of pics is genuine but it's really banged up, maybe from actually using it as a keychain where the charms are constantly rubbing and banging against each other as the car's moving.