Misaligned Die. A striking error from one or both dies not seated properly during setup, or working loose (chattering) during striking. There are two kinds of misaligned dies: (1) vertical – where the dies strike pieces one side of which is off center while the other side is perfect, and (2) horizontal – in which the dies are tipped causing them to strike wedge shaped pieces. The latter pieces lack parallelism. When dies are made it is required that the top and bottom are absolutely parallel. When they are placed in a press this parallelism is required as well. Dies tipped at any stage of their manufacture or use will result in misaligned error strikes. Collectors have abbreviated this term as MAD.

Reference:

NM19 {1991} Margolis, p 293-295.