Many error collectors and dealers alike are unaware of the dual dated double denomination error. That is how very rare they are. There are approximately twenty examples known for all combined dates & denominations, including any double denomination error with two different dates, such as a 1999 Cent struck over a struck 1998 Dime.
These can only logically happen at the end of one coining year and the beginning of another. A previously struck dime (1998) gets stuck in a hopper trap door and then the hopper is filled with planchets destined to be struck by the upcoming new year dies (1999). So it appears this rare combination of errors can only happen at the years end and if all the error circumstances are right!
There are only approximately twenty known for all dates & denominations for dual dated specimens. Normal double denomination errors with the “same” date, number approximately 900-1200 known. That’s a 45 to 60 to ONE “1” ratio.. Same date Double denominations currently costing from $800 - $1,100. You do the math! 60 x $1,000. Hmmm! Sounds like today’s prices of $6,000-10,000 are pretty cheap. There are about 10-12 “U.S. Mule” errors known that cost $75,000 - $125,000 each. Now consider the rarity of these dual dated double denomination errors. Lots of room for growth here, especially in the direction that the error market has been heading, UP!