I have a triple sticky piston extender which pushes a line of 10 slime blocks across a small (10 x 3) flat area as a sort of bumper pusher mob farm design. There is a triple piston extension to force the line of slime blocks across the surface, and then back again.

The timings, or movement, of the triple piston slime block line is creating duplicate slime blocks across the 3 x 10 surface. These blocks seem real, as you can walk on them, and destroy them, but they are not real in the sense that quitting and reloading removes them.

I'll attach screenshots that show the line of slime blocks retracted, and extended as well as a shot displaying the duplication (which seems to happen when my back is turned!).

This farm design also showed the problem of spider jockey's hanging in mid air, after the sweeper arm "pushed" the spider out from under them. I think this has been corrected, as I have not seen any more "disconnected" skeletons in this farm.