When you place a block as a mob walks in front of it, the block disappears. Refreshing the world gives the block back to you. 

Steps to reproduce: 

1. Spawn a creeper in a box. 

2. Spawn a cat next to the creeper while it is running away. 

3. Go into survival mode and give yourself invisibility.

3. Place a block in front of it as it is running away.

4. If done correctly, the block will disappear. Refreshing the world will give it back to you.