FOSS gamedev and creative worlds

I was in my mine, supervising the workers and one of them was around the -400 mark and I looked in its inventory, and I was very surprised to find a block that was labelled "IGNORE you hacker, you!" It had the texture of an unknown block, even though it clearly knew what it was. When I placed it it came up with 4 error messages, but I'm not sure what they were. Is this a super rare ultra block or something? Like the legendary nyan cat block?

No, the you hacker, you exists in the mesecons mod.

I think it's part of a piston or receiver, but I know I have seen it too in the creative inventory provided with 0.4.2rc-1.

That block appeared suddenly in my inventory labled as air

ignore = block which represents not generated/loaded blocks
air = no comment
the thing in mesecons = technical blocks you should have in your inventory

"You hacker" is a joke started by c55 regarding air, but it is also in mesecons for any of the objects you're never supposed to have in your inventory (like any of the intermediate wire shapes other than a straight length of it).

It shouldn't even be in the creative inventory for mesecons.
You can only get it when using /giveme