(I was playing around with pistons in creative but this works in survival as well)
If you hook up a rapid pulse circuit up to a piston, the piston will retract and attract. I haven't tested if this works with any block in the game but if you for example place a diamond block on the block the piston is pushing the it will sometimes push two blocks out instead of one (one is real and the other one is fake as when you try to mine it the block item does not appear).
This bug can be annoying in cobble generators as no real block is obtained from it.
MC-631 Duplicating/deleting blocks