I've had a total of three different types of backpacks since I started playing. The only one I've had a problem with items disappearing from is the hunter backpack. I made sure I moved all my items into it from the other backpack before picking it up. I had it on my back and I was running around, when I suddenly was kicked from the server. When I logged back in, to my dismay, all but a few of the items in my backpack were missing. Bottle of water, battery, and duct tape were all that stayed.