In this case (the parts belong to a Rexroth A4VG closed loop hydraulic pump) it appears that
first - the piston shoes got nicely grinded, with the shoe labyrinth pattern completely erased,
second - one and only one piston shoe got separated from the piston (and disappeared completely),
third - the piston head got smacked down to a nice round pancake shape and,
fourth - the pancake got separated from the piston body.
An interesting fact - despite all the piston
department damage the retainer plate got but minor scratches!