BROCKAGE \BROK.ij\ n. a particular type of misprinted coin
I figured that just like in stamp-collecting, botched coins would often be highly sought after collectors’ items too, so I did a bit of research. Naturally, I was on the lookout for some cool words in the process…
A brockage is what results when a pressed coin fails to exit the press when the next blank planchet enters the press to be struck. The coin that fails to exit the press is usually stuck to the hammer die (typically the reverse), and is known as a “cap” because of the form it assumed (like a bottle cap). So, when the new blank planchet is struck, the obverse side is struck with the normal obverse die, but the reverse side is struck against the “cap” (the coin that did not exit from the press correctly). So, you end up with a correctly struck coin obverse and an incuse mirrored image of the obverse on the reverse side of the coin. This is known as a brockage.
— Australian Threepence
I think it’s fun to work out auxiliary definitions from contexts like this, but just in case you’re in a hurry, here’s a quick summary…
PLANCHET n. a blank metal disc prepared to be stamped to make a coin
OBVERSE n. the primary or ‘heads’ side of a coin
INCUSE v. (of a coin) to mark with a stamped impression (also adj.)
The coin in the picture at the top of this post would be called an obverse brockage, because the ‘heads’ side of the coin is repeated on the reverse side as an incuse mirror image.