Hello, so it seems like it's been a while since anyone experienced this problem, but I did last night.
We (me and 4 others) were in an abandoned mine in Forgelight. We were fighting a horde of cave spiders (trying to get to the spawner to break it) and we all died. Everyones stuff was everywhere. We all treked back to the mine and randomly picked up each others' stuff (based on who turned what corner first). There were still spiders around, so we quickly threw each other appropriate weapons and went back at it (this time a little more systematically) and broke the spawner.
It was getting late, so we cleared out the rest of the spiders and regrouped. We healed and then everyone traded their stuff back to original owners. I'm the only one with a backpack (gift from the rest of the group), so I always carry the stacks of stuff (in this case, wood, rails, string, cobblestone, gravel, fences, dirt, monster parts, etc.). Nothing too important, mostly because I'm the weakest link in the group and I make sure they carry the good loot.
Anyway, we all had our stuff back and I organized by belt, inventory and backpack. Then we signed off for the night. The kids resumed play the next day (today), but I had to work. When I got home, and logged on, I was alone in the mineshaft and prepared to /skill warp back to spawn. I opened my inventory to make sure I had the 10 redstone and I noticed my backpack was empty.
I am absolutely certain I did not die, I'm absolutely certain I was not in combat when I logged off, and I'm certain I had a nearly full backpack when I logged off. Note: during play last night, there were times when one or more of us had to log off for some reason or another (including me) and that didn't seem to bother the backpack. This seems to be some sort of overnight reset or something. Also note, I've only had the backpack for a couple of days.
Sorry I don't have any video (nor do I know how I would make a video since I would have to log off and wait for morning). I will leave something in my backpack every night and see if I can duplicate the problem.
darthdoom