If you go to the world border (or shrink it down so you're next to it) you can place one slab next to the world border and then another slab next to that slab but on the opposite level of that other slab, and you push the second slab with a piston you can create a block with two different kind of slabs, when you then try to break that block only the second slab will break and the first slab will remain, you can still jump on it and it still has collision, but you can also break blocks through it, if you die the slab will then apparently disappear, however, that is just a visual bug as the slab remains there.

Video depicting the bug: https://youtu.be/o92CYFBLerE