A brockage happens when another coin is struck into another coin or planchet creating an incuse and mirror image. It can be full-on or off-center. Further note: some people have tried to imitate this error by placing a coin on top of another and striking it with a sledge hammer but they do not show the distortion and debt of indent nor are they a mirrored image.
Brockage strikes can be worth $50 when it involves a partial indent on a common cent or nickel and climbs in value as the indent covers more of the struck coin. Type coins and larger denominations can be worth hundreds of dollars with one 1874 $1 centered brockage sold for $30,550.
Images courtesy of Heritage Auctions