I had played 14w26a yesterday, and I was now using 14w26c for the first time. Also, when I left the world for the last time yesterday, I was in spectator mode (looking for interesting caves)

What I expected to happen was my world to be exactly the same
as it was yesterday

What actually happened was many things I had placed (mainly stone bricks, some fences, furnaces, crafting tables, wood slabs, prismarine bricks) had disappeared, leaving gaps in all my stuff

Steps to Reproduce:
1. Place things (blocks listed above) in a default-gen world in 14w26a
2. Leave the world while in spectator mode
3. Open the world in 14w26c
4. look at where you had placed the blocks before.