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 touches the block, half a heart of damage is taken. 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 1 cacti block.

Since Secret Friday Update 6, cacti can only be placed on sand with no horizontally or vertically adjacent blocks, solid or liquid. However, blocks can be placed diagonally next to them. When the spot they are placed in becomes unsuitable (e.g. a block is placed next to them), they collapse into a cactus block and can be re-placed. As well, mining cacti no longer harms the player.

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

New cactus after the 23nd, July update.