X, Y, and Z parameters for command blocks require the other two values to work

Falling block entities can't have a block texture that's not sand or gravel in 1.5

A redstone block can power a piston 2 blocks below it

Nonreactive versions of stacked mobs appear when a spawner is set to spawn them in a specific location

The hopper is in the wrong category

The block under a daylight sensor doesn't get powered at any time.