So, I'd just downloaded the latest snapshot for MC. Deciding I'd rather wait for the official update because of lack of door sounds, annoying world loading and McPatcher not liking it, I deleted minecraft.jar and forced an update. SO I went to play SSP for a while. When I began, I was not outside my mansion. I was in a seemingly untouched forest, at what must've been my original spawn. All the stuff in my inventory was gone. By "all the stuff", I mean loads of Cobble, Planks, Glass, Buckets, Torches, Coal, a full set of Iron armour and the only Diamond tools I own. Having written down my home coordinates, I managed to find my way back, but there just wasn't enough to replace what I'd lost. Luckily I had a backup file. I can't remember actually doing anything since I saved it, and if I can't remember it it's not important. Has anyone else ever had this happen to them.
I went to play my SSP world in 1.1 when the world was in Anvil, so I spawned inside of a rock in a little pocket underground, and I broke every one of my picks trying to get out. Then I finally realized I was running the wrong version. So I swapped the .jars and reloaded the world, and my picks were still gone, but everything else was untouched (thank god).