Didn't see it already bugged. If you have the extended portion of a piston under a block dispenser and activate the dispenser it "pulls" in an unusable (unnamed) item and breaks the piston.
Only found it as I made the mistake of putting my pistons diagonally from my dispensers on my automated minecart(chest) emptying system. These dispensers were only meant to be pull and replace the rails so my pistons can push the items off the track area. Fixed my problem by putting a block in front of a sticky piston instead, allows for enough distance so my pistons are not powered by the dispensers.