Need help on this jfk coin

Hi first time with a post here have been reading for 2 years I think so be kind when I screw up because this is my first post . Here I have a uncirculated set 1776- 1976 bought on a on line auction for 15.00 but when I really look at it I saw the error on the half dollar needless to say I can't find anything like this any ware. I don't even know what to call it. So all help is need thank u hope I can figure out how to attach the pictures

 

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those lines on the half dollar looks like the machine that seals the plastic stuff and the perforation/sealing machine hit the edge of the coin.
I've forgotten what that is called though.

 

That's exactly right. I have a couple of these myself.

 

It is considered damage and not a mint error.

 

$15 for the 3 piece silver mint set is.a decent pick up!

 

thanks