When pistons are placed in a position where the head is facing south, the head of the piston appears twice on the upper and lower faces of the block. On the upper face, it has double heads facing the same direction, on the lower face, it has one head in each direction. Does not show on the sides of the blocks or in other orientations. Occurs in both powered and unpowered states. 

In the photos attached, the arrow made out of redstone lamps points towards the northern direction. 

Doesn't appear to affect gameplay. However, when under the pistons which are facing southward, it may confuse the player as to which direction the piston will push/pull, since it is completely symmetrical when unpowered and when the sides are not visible.