I know this isn't a coin but I collect tokens and they are made the same way. I came across a huge batch of these St. Mary's, PA parking tokens and out of a bag of about 1000 I found one odd one. It has the appearance of being in a bevel ring with a prominate rinsed lip on both sides. Under a loupe it is one piece and not an attached ring. It is also 0.65mm small than all the others. It doesn't show up well in the scan but I attached a scan of the error and a regular one.

