Wow. Look at this neat nickel. One side is fine, the other is mushy and messed up. It is a struck through die cap error coin. These can be valuable. See below.

CoinQuest thanks respected eBay seller amyluvsjasper who has the piece in the picture is currently at auction.

Coins are created when two hardened pieces of metal known as dies come together with great force on a coin blank, or planchet. Sometimes a coin gets stuck on one of the dies, and this coin becomes a die cap. You can see a picture of a die cap at this CoinQuest link. If the die cap stays in place for any length of time, it strikes other coins, and these coins are called brockages or, more explicitly, struck through die cap errors. That is what amnight has in his or her collection, and that is what amyluvsjasper is selling at auction. There is an avid collector following for minting errors, and a very limited supply of collectible errors. This sends the price up. You can see a picture of another brockage at this CoinQuest link.

Mike Byers is a noted authority on error coins, and he wrote the book on error coins. His catalog lists these approximate values for US struck through die cap errors:

Large cent: $1000
Indian cent: $1000
Lincoln cent: $300
Shield nickel: $2000
Liberty nickel: $2000
Buffalo nickel: $2000
Jefferson nickel: $100
Barber dime: $2500
Mercury dime: $1500
Roosevelt dime: $100
Washington quarter: $300
Kennedy half: $800
Ike dollar: $2000
Small dollar: $1000

These values, obviously, are tremendously approximate. Seek guidance from knowledgeable collectors or dealers for your particular example.