Description:
I opened Minecraft that has only been minimized, I entered my Creative world in the latest Snapshot (15w51b) and I started descending like if I was floating downwards, still able to move. But then I got to the floor of my flat world and I sink through the blocks like if I was in Spectator Mode, but I wasn't so I started flying and everything was like a normal creative mode fly. So I restarted my launcher and started recording. Again then the same thing happened when I opened my world. Please note this in not the first time this happens to me.

What actually happened was...:
This is explaind in the description

Steps to Reproduce:
There wasn't a procedure, it just happened when I opened a world

I expected a normal Minecraft Physics
where you can not go through blocks