So I was trying to stop a basic flying machine, in a superflat world, with a redstone block.

Placing the redstone block between the piston and the slime block made the redstone block disappear and the machine continued to go forward, but the rear of the machine duplicated itself and stayed behind.

The blocks are visible, solid (one cannot pass through) but do not drop anything when in survival. I tried pushing them with a piston, which worked, but when I deactivated the piston the blocks disappeared.

I succeeded in replicating this bug four times.

Here is the youtube link of one instance of the bug

https://youtu.be/ellMtRjfN-4

(sorry the video exceeded the 10MB limit for the file attachment, youtube was my solution)