Used pistons to make a redstone block completely disappear (see pics).
Basically, the game rendered the movement so fast that the block became invisible and invincible (I couldn't break the Redstone block itself, first the contraption would need to be broken).

Pretty easily duplicated, and I don't see this as game breaking or anything, but I figure it is probably a bug worth noting.

(This was also single player, I have no idea what this does in SMP)