In 1999 the Mint had to provide General Mills 5,500 Sacagawea Dollars for an agreed promotion. In the promotion General Mills would place 5,500 Sacagawea Dollars inside every 2,000th box of Cheerios. In 2005, collector Tom DeLorey discovered that some of these 5,500 Sacagawea Dollars had a special detailed design on the reverse of the coin. This different design had detailed veins in the eagles' tail feathers. Uncirculated and including Proof coins lacked these fine details on the eagles feathers.
After the initial discovery the hunt was on and collectors searched to see if they had this scarce design referred to as the Cheerios Dollars since the coins were only being found inside Cheerios boxes. After several years only several dozen examples were actually found. Who knows how many are still out there, but finding one would definitely be an incredible find.