Found this while searching through pocket change, and at first thought it was post mint, but then thought differently when I looked closer. The indent or mark on the 6, seems to go the whole way around the six, but the six still stands out plain as day. My thought would be if it was post mint, wouldn't the 6 be taken off or indented, not like it is? Can't seem to be getting the words I want out, lol, but I hope the pics will do the talking for me. So all I really want are a few answers. One, is it a mint error and what would terms of value be? And if it isn't a mint error, how would something like this happen? Thanks ahead of time, here are the pics:
Phoenix
1960 D cent error? 
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