Coin are often found in any condition possible to the imagination. When it comes to coins anything can happen and everything has been done to them. Below are three examples of coins with purposeful or accidental damage, but no matter how it happens this damage lessens the coin's value.
1893-O Barber Dime might have been damaged in a gravel driveway and was ran over several times.
The above Lincoln Cent might have been damaged by a stick pin. It's a shame because it's an authentic 1914-D Key Date, but this still lowered the value of the coin.
The above is an image of a coin that was cut on the edge with a knife. Some people just get board and play with knivesa and coins, so this is not a mint error.