I thought when I would switch versions it would make a smooth transition as long as I had no new-version items in an older version world.

When I opened Minecraft and switch from, say, 1.7.5 to 1.7.9 every single item I hold in my hand or in my chests and furnaces and such just disappear. It was like they weren't there. Even the items on my horse, like a saddle and armor, and the armor on my body was gone.

Steps to Reproduce:
1. Open Minecraft.
2. Choose version in "Edit Profile."
3. Play for a couple minutes.
4. Decide to change versions.
5. Save and Quit Minecraft.
6. Open Minecraft.
7. Change to a higher or lower version/snapshot in "Edit Profile."
8. Go into my world.
9. Discover all of my stuff is gone.
10. Repeat until frustrated.