Touching a coins surface with your bare fingers can leave oils that will show on the coin as time passes. You won't usually notice these when they happen, but later after the oils have already damaged the coin. It is good practice to hold a coin by it's edge and always wash your hands with soap and water before handling your coins.
Photo By LincolnCentsOnline
There are many examples of counter stamped cents circulating. Some will have letters or numbers punched into them, or a club logo, etc.
To see more counter stamped Lincoln cents Click here
Sometimes referred to as the "Kissing Presidents Penny". This was made by punching an impression of Kennedy's head next to the bust of Lincoln. These were sold as novelty items and have no value to collectors.
Sometimes referred to as "Hobo Coins" these are genuine cents that have been hand carved to show a new design. They are a form of art and are not mint errors.
To see more carved Lincoln cents Click here
Photo By LincolnCentsOnline
This is what happens to a Lincoln cent when it falls into a commercial clothes dryer tumbler. Sometime they get stuck and tumble a long time. This gives them the look of a spooned coin, one that has been tapped with a spoon. The details appear wider and smoothed over. Many times the edge will roll over the faces of the coin and give it the look of having a ring attached to it. The damage varies on how long the coin was in the dryer. They have no added collector value.
Coins can sometimes have the last digit in the date appear to be slanted. This is usually caused by a vending machine or coin wrapping machine.
Photo By Lincoln Cents Online
When a proof coin has damage from any contact, it is called an "Impaired Proof" and loses considerable value to collectors. Proof coins are not issued for general circulation, but once in a while one ends up there. This 1976-S was found while searching circulated rolls from a local bank.
As the story goes, back in the 1950's - 1960's when it cost a dime to use the pay telephones and the really cool stuff was always in the dime gum ball machines, kids would file a penny down to match the size of a dime. Many an unsuspecting machine was taken advantage of this way. If you look at the edges of the coin and examine the dime overlay, it becomes clear that is what has happened in this case. You can even see how the face of the coin was filed down to match the thickness of the dime. Some cents do get minted on the wrong planchets but many are like this one!
Sold as novelty items, coins are hollowed out and made to come apart so there is a secret compartment inside. Sometimes two separate coins will be fitted together to show two obverse or two reverse sides on the same coin. There are even examples where two separate denominations are used to make it appear as if the coin has changed value simply by turning it over.
Some people have wondered if altering a US coin is illegal. As long as there is no intent to defraud someone, as in counterfeiting, it is not illegal.
The coin above is an older undated wheat cent that shows the reverse on both sides.
Photo By R.S.Cooper
Photo By R.S.Cooper
Photo By R.S.Cooper
Obviously mutilated by tooling or by someone attempting to create a design on the face of the coin.