Using sticky pistons and slime blocks blocks ghost blocks are sometimes generated

I was creating a secret passage way, when I noticed that when I tested it there was an extra block. This only happens about 1 in 10 to 15 times, but it makes an extra ghost block of the block when it works. I have tested it with block of diamond, bookshelf, and sand with a block under it. It has not failed with any block I've tested so far. It does not work when I increase the redstone repeater connected to the piston connected to the slime block. The video shows the creation of ghost blocks, the screenshots show the machine.
Edit: This also works in survival