shadows under slabs that have solid blocks on top of them

if you make a floating 9x9x2 of any (solid not traslusent) block then delete the lower middel block. then replace the lower middel block with a (half block) slab (place it under the top middel block). just under the slab will be shadowed (dark black) this affects all gamemodes

it has had this glitch since i got minecraft. (1.5)
but i have played minecraft since 1.4.7 from a frends computer
i play off line 99% of the time so i most likely mised some updates.
i am vary vary unhappy adout ths glitch