I found a 1965 dime that looks like it is "mushroomed". The whole rim is missing, like a groove around the whole thing. The top and bottom are still there. any thoughts?
PMD, and to answere your question, probably some angry man with a grinder/revolving sander in his garage. Maybe a science experiment? ah the possibilities!
This happens when a coin is left in an acid bath too long because the copper is a softer metal than nickel it is eaten away first leaving the harder nickel.