So the setup is this: you place/bring a shulker on the ground, you then place a piston facing the shulker 1 block up and to the left of it. Then place a lever on the piston. Flick the lever, and place sand on the piston arm.

 I, unfortunately, don't know how to reproduce this glitch, but fiddling around with the piston and such can cause the gravity-affected block to... how do I describe this... um, imagine that you can only place blocks on an integer x or z level. but this glitch lets it be        ± 0.5 ± x (or z). It's a bit compilated but meh.

Note: the block that was put off-center by ± 0.5 blocks will be destroyed when it gets a block update (e.g placing a block next to it). 

I'll make a screenshot when it happens next.