When you connect a redstone block to a piston it activates, but when you add another redstone block on top of the redstone block that activates the piston and break it the floating one will keep the piston activated.

How to reproduce

Craft a piston and 2 blocks of redstone
Then connect the block of redstone with the piston to activate it
Then add 1 block of redstone on top of the first block of redstone
Then break the first one you placed and the second block should be floating and the piston will still be activated when it should deactivate.