Barrier

The Barrier is an invisible block added in 14w05a. The block appears transparent, but does not act as a transparent block. Buttons, redstone, ladders, ect. all can be placed on the block. The block in the player's inventory shows up as a red box with a slash through it, but when placed is completely see-through.

Trivia

 * Like Bedrock and Command Blocks, barriers cannot be pushed by pistons and can not be destroyed without Creative mode.
 * When you break barriers, lava particles come out.