There is a newly discovered mint error 2021-W quarter ounce gold American Eagle, struck with unfinished dies! PCGS just certified a number of examples. The 2021-W date/mintmark combination is only used for striking proof gold 1/4 eagles, but these discovery pieces have business strike surfaces. Some 60+ pieces were discovered by the discoverer, Gerald Medel, upon searching thousands of 1/4 gold coins (see article link below for more details.)
An error of this type would have occurred do to a skipped step in the die production process. When the blank piece of die still was put into a hubbing press and impressed with the quarter ounce American Eagle gold design, it would at that point only have had business strike surfaces.
However, in order to give it surfaces for striking proof coins, it would then receive a treatment process (polishing, laser etching, etc) that would give the die's business strike die fields, the frosted devices and mirrored fields so that it could then be used for striking proofs.
Instead, this die pair missed that step entirely, resulting in a die with the 2021-W date and mintmark, but with business strike fields instead of proof fields.
Doubtless, because the die pair then had "business strike surfaces", a press worker accidentally used the die pair for striking regular, business strike bullion coins, unaware that the obverse die had the "2021-W" date and mintmark combination that is reserved for the proof only issues.
This error type also happened in 1999 for some of the 1/10 and 1/4 ounce gold 1999-W American Eagle issues. Some 6,000 or so of each coin were struck with unfinished proof dies, resulting in a coin with the dies of a proof coin, but the "finish" of a business strike.
A fascinating new mint error, and one to watch out for as collectors and dealers process their new 2021 1/4 T-2 gold American eagle coins. Doubtless more are waiting to be found.
Below is the link to PCGS's article about the coins, which provides more information. https://www.pcgs.com/news/regular-strike-quarter-ounce-2021-american-eagles-with-w-mint-marks?fbclid=IwAR3PRGqWpmed4LavObtHGj93_IFD8eG5voyVk_qDHmsnH-PHnx0Le3g7sVg
(Image Courtesy Todd Pollock)