If you make stack of 2 blocks then destroy the bottom block and place a beacon where the bottom block was the beacons bottom turns green. After finding this bug I tested it 5 times and it always worked.
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I just tested and realized if there is a block on it the top piece of obsidian turns the color of the block under it
I think this is Minecraft trying to reduce rendering computations, leading to it no rendering the tops of blocks (and things inside the blocks) when something is on top of the blocks.
Exactly!
shufboyardee
It resembles issue MC-102, which was fixed in 1.4.2 or 3; but it definitely happens with this block in 1.4.5 (see screenshot).
This is because the internal structure of the beacon's top faces are considered the 'top of the block' and in Minecraft processing power is reduced greatly by only rendering 'visible geometry'. That is, underground nothing is rendered except for the walls of the caves inside it.
Summarized this bug to MC-1038.
Its Not Green! Its Invisible!
Is this just the top piece of the obsidian going clear or does it go green? also, does it work with just placing the block ontop?