When trying to place the top half of a half slab while at max build height, the block will "disappear" from the inventory, and can only be retrieved by placing an item in the slot the slab was in, or by picking up the stack of slabs, that the slab came from, while being informed that the max build height has been reached. Workarounds include placing a full block instead of half slabs. This does not prevent, the placing of upside down half slabs.
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If the max build height block is a layer of snow, placing any block, not just a half slab will produce the same results as above, with the same message, instead of destroying the snow and placing the block.