Nether Portal block in inventory renders as a cube if no portal blocks exist in the world.

When a nether portal block is in the player's inventory when a world is loaded (through /give, and inventory editor, etc.), it will render as a solid cube, with 6 sides each showing the nether portal texture. The portal can be given in game as long as it is not placed. As soon as the portal block is placed the block in the inventory begins rendering normally, and any additional portal blocks obtained will render correctly. The game must be completely shut down in order for the portal blocks to render incorrectly again. If a portal block already exists in the world, the portal in the inventory will render normally.