If you log in to a world and log back out before the world is finished loading, and log back into the same world, your inventory and any other things that hold inventory (chests, furnaces, ect.) will no longer have the things that they had in them. Plus, furnaces that were burning will burn forever until updated.
Related issues
duplicates
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Why don't you simply restore your backup ?
I am also angry at myself because I am the idiot that didn't make a backup before I loaded the snapshot. Sadly a lot of others probably do it too...

Did you downgrade to 1.7.4 after loading the snapshot ? If so --> MC-45009
Or did you load the world in the snapshopt, close it, load it again in the snapshot ?
I had reloaded it like 20 minutes after I fired up the snapshot for the first time.

So... you did downgrade to 1.7.4 at one time with your world?
MC-45009
Downgrading is, was and never will be officially supported.
Nice to see it has happened to someone else...
I am extremely pissed. I get that snapshots are going to be buggy, but they should not be able to delete everything in your world. Shame on you Mojang, guess I have to restart EVERYTHING!!!