Put the summary of the bug you're having here

What I expected to happen was...:
My and my friend were on a server, we had a lot of lag. We just updated to 0.7.0. We typed in the seed known as "jpgaming". We went straight down from spawn to find diamonds. We were trying to smelt the iron, and it was laggy, and we did what happened in the reproduction steps. I'm not sure if this was a bug from the update because we didn't play any of our worlds by ourselves.

Steps to Reproduce:
1. First, put something in the top square.
2. You shouldn't be able put the item back or put something in the bottom square.
3. ...