State quarter errors are mistakes made on coins from the U.S. Mint's state quarter series, which was launched in 1999. Types of state quarter errors include missing design features, improper coatings, wrongly sized coins, off-center strikes, blank coins and die cracks and rotation errors.
Some Wisconsin state quarters produced by the Denver mint in 2004 carry an extra leaf in their design, an error that increases their value to collectors. Some coins with this error sold for over $1,500. As of 2014, off-center strike errors increase the collectible value of a state coin to over $100. Coins missing their outer layer only have a copper coating on one side and may be worth between $200 and $2,000 to collectors.