When a client places a block directly on top of another block it doesn't register on the host's device, therefor not existing at all.

After I was done with my house my friend (host) came and looked and said "why are there holes in your house",and half the block I had placed weren't there.
The blocks should be there.
The blocks weren't there.
The blocks that should be there were not there.

Steps to Reproduce:
1. Place a block directly on top of another.