1998 P Nickel Brockage

It happens when two coins are involved. First a coin is struck and when it is ejected, it does not eject properly. That struck coin will find its way back between the dies and the dies will strike this improperly ejected coin into the next blank that was fed into the collar. Typically the reverse of the improperly ejected coin gets then pressed onto the obverse of the blank and well it will be a mirrored strike. Most of the time only part of the blank gets this error strike. On some occasions it is possible that the improperly ejected coin finds it's way back perfectly and does a perfect brockage in which the image of the improperly coin gets pressed complete covering the obverse of the blank planchet. The result is ofcourse an incuse press result. Some occassion the improperly ejected coin will flip and the other side of the coin will be incused onto the blank. Really neat and cool looking error!
http://www.errorsoncoins.com/nickels/brockage/