Scott Sery writes:

There are five groups of U.S. proof error coins that 'escaped the Mint'. Below I give three examples with brief descriptions. The most famous hoard is the group that was auctioned by the State of California, after being examined by the U.S. Secret Service.

1. The SF oil pan hoard- in the 70's the forklifts that were used inside the Mint were serviced outside of the Mint. Employees would place proof error coins in the oil pans.

2. John Devine's (Lonesome John) hoard- John had many proof error coins from the SF Mint, including an Ike Dollar in proof, struck on 3 nickel blanks. I purchased this clover leaf and sold it to Dr. Berry.

Dr. Berry assembled the finest Ike Dollar error collection in the 70's and eventually donated it to the ANA.

3. The SF safety deposit box hoard- the most famous hoard known of U.S. proof error coins that escaped the Mint. A group of San Francisco Proof Errors were discovered in a safety deposit box. They were unclaimed and subsequently auctioned by the State of California, after being examined by the U.S. Secret Service, after determining that they were legal to own.

4. A 50 year raw collection that surfaced around 2015.

5. Another 50 year raw collection surfacing around 2020.

Below are three links to a U.S. proof error coin from one of these groups mentioned above:

This mated pair was recently authenticated and certified by NGC and is from a 50 year old U.S. proof mint error collection that recently surfaced. This mated pair belongs in an Eisenhower Dollar collection or a collection of major mint errors:
https://mikebyers.com/6329750-003-004.html

This deliberately struck unique and exotic Bicentennial Quarter error is in gem condition with amazing eye appeal. It is pedigreed to John Devine "Lonesome John," a famous pioneer in the field of mint errors and was just certified by NGC. It was previously unknown to the numismatic community and is just now being offered for sale:
https://mikebyers.com/6606002-001.html

This unique 1970-S Proof Quarter from the United States was overstruck on a 1941 Quarter from Canada and was intentionally produced. This mint error was originally discovered in the group of San Francisco Proof Errors that was auctioned by the State of California, after being examined by the U.S. Secret Service. There is a significant amount of detail on both sides showing the design of the Canadian Quarter. This is one of the most fascinating and intriguing proof mint errors ever discovered:
https://mikebyers.com/45587309.html

To read the earlier E-Sylum article, see:
NOTES FROM E-SYLUM READERS: FEBRUARY 2, 2025 : Error Coins Stolen(https://www.coinbooks.org/v28/esylum_v28n05a12.html)