Cactus

Cactus is a block that is only available in Alpha mode. It was first seen in the Secret Friday Update 5, about Minecraft Alpha version 1.0.5_01. The block has one texture put on all sides of the block, one put on the top and bottom, and an unused texture resembling a tree crosscut indicating the inside of the cactus. It can be broken easily without any tools.

It appears on Sand in vertical of stacks that can be 1 - 3 blocks tall, 1x1 wide. They are somewhat rare, and appear in normal and winter themed maps. When the player clicks the block, half a heart of damage is taken, even if the player is not using their hands. This can be avoided by beginning to mine on a different block, then moving over to the Cacti block. Half a heart of damage is taken for each Cacti block walked on; for the player and mobs. This makes it useful for traps, such as a hallway with Cactus block floor. If any mobs to walk into the hall, the floor will kill them quickly. When mined, it can drop 2 cacti blocks, allowing it to be farmed to get much more blocks in your inventory than was original plant amount.

It is possible that cactus blocks will serve more purposes in the future.