Double Die Errors: These occur when a coin is struck more than once, resulting in visible doubling of letters or numbers. For instance, the 1999-P Delaware quarter has a well-known double die error where the word "Delaware" appears doubled.
Missing Inscriptions: Some state quarters were minted without certain inscriptions, such as the state motto or date. A notable example is the 2004 Wisconsin quarter missing the “E” in “We Grow,” making it highly collectible.
Off-Center Strikes: When a coin is not perfectly aligned during striking, it can result in part of the design being off-center. This can make the coin visually interesting and valuable, such as the 1999-S Georgia quarter with a significant off-center strike.
Wrong Planchet: Occasionally, quarters are struck on incorrect planchets, like a dime planchet or a foreign coin planchet. These errors can make for highly sought-after collectibles. For example, a quarter struck on a nickel planchet is a rare find.
Die Cracks: As dies age, they can develop cracks that get transferred to the coins. These die cracks can appear as raised lines on the coin’s surface, adding unique character and potentially increasing its value, as seen in some 1999-S quarters.
Overstamped Errors: Sometimes, state quarters are mistakenly struck over coins from earlier series. These overstamped errors can be particularly rare and valuable, such as when a state quarter is found with remnants of an older coin design.
Mule Errors: A mule error occurs when coins are minted using mismatched dies. For instance, a 2000-P South Carolina quarter was found with a reverse die intended for the 1999-P Delaware quarter, creating a unique collectible.