When 2 observers / repeaters set to 2 gametick delay power a piston with a block on its face, one right after the other, the piston ignores the 2nd pulse. For example, a sticky piston will spit out the block. 

See the attached screenshots for the setups I used. Also, here's a gif to compare it to a 4 gametick pulse given by a repeater. https://gfycat.com/BronzeScientificKilldeer