When you right and left click a block at the same time, it is replaced by the block in your hand. This happens even if the block is in the air with no other blocks around.

What I expected to happen was...:

I expected the block to break and only get replaced if there is another block to place on.

What actually happened was...:

The block break sound occurs and the break particles show up, but the block is replaced by the one that you are holding.

Steps to reproduce:

1. Create a new single-player creative world
2. Place a block at least 2 blocks above the ground
3. Get a different block from the creative inventory
4. Right-click and left-click at the same time
5. The block breaking sound effect and particles occur and the new block is placed where the previous one was

This can be viewed in the attached video.