Put the summary of the bug you're having here

I loaded a old world in 14w28a (last loaded in 1.7.4). After playing a while in the snapshot, I closed the world and opened it in 1.7.4 again. All the containers have gone empty or weird in 1.7.4. This includes signs, chests, hoppers, and your inventory. items in chests and hoppers all dissapeared. My inventory was cleared. Messages on signs have all gone weird.

what happened to the signs are included in the attachments

Steps to Reproduce:
1. open minecraft 1.7.4, put a sign and write some stuff on it. also put a chest and put some stuff in it
2. reload the world in 14w28a, after it gets rendered, close it and reload it in 1.7.4
3. all the contents in the chest should be gone, and the sign should be messed up

so far it doesn't affect command blocks