Blocks can be placed on the other side of rapidly extending pistons.

Blocks can be placed on the back of rapidly extending pistons.

How to reproduce:

1) Place down a piston facing you.

2) Connect it to a fast redstone clock (but make sure there is air directly behind the piston).

3) Aim at the middle of the piston, hold a block in your hand and hold right click (or whatever key you use to place blocks).