What I expected to happen was that I would walk over the piston's extended arm.

What actually happened was that I fell through it occasionally.

1. Extend the piston (power it).

2. Stand on the piston's face.

3. Walk towards the larger block section of the piston (the section with the furnace-like texture) while staying towards the center of the arm.

4. After a few tries you should fall through the extended arm.

Note: Having your viewpoint higher than 90° (higher than looking forward) seems to make it more likely to fall through.
Also happened with blocks attached (sandstone).