I was making a machine with Redstone and happened to use a combination that revealed this bug. When a redstone repeater is pointed into a air block above a piston it acts as though a real block were there, completing the circuit and activating the piston. While this is acceptable if a real block was there, in the screenshots below it's obvious that no such block exists.

Also on the subject of pistons:
If any solid block is pushed by a piston upward and then proceeds to have a signal go through it, the piston head receives power and does not retract when the base of the piston stops receiving power.