When placing a Redstone block on top of a piston and then a 2nd one above that same redstone block the piston will enter a state of permanent activation "even when the redstone blocks are removed." If a piston "facing up" has a redstone block on it and is triggered by a redstone signal the piston will then stay permanently powered as well. 

Steps to recreate
1. Place Piston
2. Place "Redstone Block" above "Piston."
3.Place second "Redstone Block" on first "Redstone Block."
4. Remove First "Redstone Block" then remove second "Redstone Block."