When powering a piston or sticky piston, it shows the piston head as a full block (on horizontal pistons it is smaller than a full block).
Related issues
is duplicated by
Attachments
Comments

I've noticed that all pushable none solid blocks (tested with slabs, stairs, fences, gates and trapdoors) appear as full block when pushed by pistons
Confirmed for 14w17a
Confirmed for 14w18a
Confirmed for 14w20a and b. As of 14w20a, piston head doesn't appear anymore as a full block but there are now two heads.
Confirmed for 14w21b.