There are numerous minting varieties, but not all are errors. Some can be chalked up to wear and tear on the minting equipment. Collectors are examining the State quarters much more closely, not realizing that the Washington quarters have always had a lot of minor minting varieties. For instance, die cracks are so common on the quarters that they have no collector value. Die breaks, however do have some value.
The most significant find so far are the 180 degree rotated reverses on some of the Pennsylvania quarters; which have been valued at more than $500 each.