Minecraft1.8.8

Introduced in 1.8, these allow you to create "flying machines" or moving "pieces" of a base because they stick to adjacent blocks when pushed by a piston. Iron Trapdoor:

Years later, long after the server’s RAM was reassigned and the last player logged out, a dataminer found The Anchor’s backup on an old hard drive. The checksum matched. The world loaded in seconds. Minecraft1.8.8