How Does An Error Like This Happen?

Thats insane. I don't know how it was created but for sure a logjam in the minting chamber.

Oh my!

Talk about a pile-up!

It's there - under Bonded Coin Errors. Here's a link:

- Part VI: Striking Errors - Bonded Coins

Spence pretty much described it...Here's my understanding of how this can occur (from a conversation with an error coin dealer several years ago - not Fred Weinberg, not Jon Sullivan).

As I understand it:

- An initial coin is not properly ejected from the die chamber after it is struck (i.e., remains in the chamber, either wholly or partially); if the collar was not properly in place when the dies struck the planchet, the coin may be struck off center or be a broadstrike.

- The planchet feeder mechanism malfunctions and feeds one or more planchets into the chamber.

- The dies then strike the multiple planchets in the chamber and, because of the high pressure involved in each strike, the coins are bonded/fused together.

- the improper feeding and striking of planchets can repeat causing a bound pile-up - what appears to be shown in the OP's image.

Pretty cool error, what they're calling the reverse looks crazy!