Saving and loading pistons in motion, using structure blocks, can cause "Empty" blocks.

How to replicate

1. Build a redstone circuit setup as shown in the video below.

1. Activate the redstone clock with a 1-tick pulse. 

1. While the circuit is active, save the entire setup with a structure block.

2. Use a structure block to load the structure.

3. One of the pistons is missing, and has not been replaced by an air block. 

The "Empty" block is not an air block or a void block. It is literally empty. Blocks can not be placed inside of it. Entities can pass through it, but blocks, liquids and piston extensions cannot. It cannot be mined, but nearby TNT explosions will replace it with an air block.