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)
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After consideration, the issue is being closed as Won't Fix.
Please note that this is not the same as Working as Intended, as this bug report correctly describes behavior in the game that might not be the intended or desirable behavior, but it will not be fixed right now. Sometimes, this is because the issue reported is minor and/or impossible to change without large architectural changes to the code base.
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