Pistons become insubstantial when retracting

When a piston is retracting, the body of the piston becomes intangible for a moment. This means that the player can be pulled through it by the piston head. It also means that blocks that need to be attached to a block (like torches or redstone) placed on top of the piston will pop off when the piston retracts.

This video by Sethbling provides some more detail: https://www.youtube.com/watch?v=0b7A4m2SFSQ