If you are standing on a bottom half slab, you are able to break it by mining without looking at it so long as you are far away enough from other blocks.

update: it does the same thing with string, enchanting table, item frames, flower pots, chests, trapdoors, scaffolding, lectern, brewing stand, wall grindstone, any non full block from what I’ve tested.

How to test: place the block, look away and down, then mine. You can use something similar to what I built in my world. (Seen in image)

I also noticed the outline is on the block below you even if you are not looking at that block so long as there are no other blocks close enough to break.