I was experiencing some issues with various bugs in the new 1.8 release (I already reported them) and was going to play in the previous version 1.7.10. Upon opening the world again, my inventory was wiped clean and so was the first chest I saw and opened. I went back to using 1.8 and it was all still gone.

I expected to have all my inventory and everything to work right.

My inventory was gone and so was everything in the chest I opened.

1) Open a world from before 1.8 with 1.8 and play with new items a bit.
2) Close Minecraft and and open it again using version 1.7.10.
3) Open the same world and check inventory and any chest(s).