All items disappear from both inventory and chests when switching from current release to 14w18b and back.

I was having a persistent problem where my framerate would drop to zero after about two to five minutes of gameplay, which required a full restart of Minecraft to fix even temporarily, after which the game would slow down again after a short time. I tried turning off the 14w18b snapshot to see if this would fix the problem, only to find that all items were gone from my inventory.
After respawning back at my home base, I found the base intact, but every container in the base was completely empty.