The contraption (image) involves launching anvils from the bottom (dark-grey blocks) to sit above the surface (white blocks). It's designed so that once the anvil stops rising (traveling upwards), its falls (downwards) less than one block before resting (stationary) on the surface. Despite this minimal falling height, the anvil still becomes damaged.

I believe that the rising anvil registers as a Falling Block, meaning rising distance essentially becomes falling distance. The issue/bug is that moving anvils are missing an exception that prevents rising distance from being classified as falling distance. This may apply to all Falling Blocks.