Common, scarce and rare U.S. Error¹ State Quarters (also known as Error Statehood Quarters) at Mintmark.com; includes links to Washington Quarters and non-error Statehood Quarters. These coins are very popular² with collectors—especially in high grades (condition)—and due to scarcity, many are quite valuable! Please note: Some offers may include gold-plated, silver-plated, colorized or otherwise embellished (that is, intentionally altered) Statehood Quarters which are not official U.S. Mint products, and though they are collectible, they should be viewed as numismatic novelties having little or no true numismatic value.
¹ Errors made at the U.S. Mint; a mistake or mis-strike (often spelled misstrike).
² For examples, especially the 2004 Wisconsin Extra Leaf and the 2005 Minnesota Extra Tree.
". . . The 50 State Quarters program was put into effect in 1999. Each state would get a design put on the reverse. The obverse was changed so that the date would go on the reverse. Five quarters are released every year through 2008. Legislation in 2007 extended the program to the District of Columbia, Puerto Rico, and America's territorial possessions; these six additional coins were released in 2009 . . ." — Washington Quarter (U.S.) at Wikipedia. Edited and (or) revised by Mintmark.com