Typically, proof coins are struck with a mint mark indicating their origin from a specific mint, such as "S" for San Francisco.
The double die obverse error is a fascinating phenomenon where the die used to strike the coin is misaligned, causing a doubled image.
For the Bicentennial quarter, a notable double die obverse error features a doubling in the date and the drummer’s image.
A clipped planchet occurs when the blank metal disc used to strike the coin is not properly cut, resulting in a coin with a missing portion.
Such errors are rare, and when they occur on a proof Bicentennial quarter, their value skyrockets.