My daughter created a new world on her profile (survival, peaceful, didn't change any other options) and I was playing with her on split screen... After grinding for about 12 hours (mining resources, building a cool base, building farms for about 6 different mobs & all the food/honey etc), I built a nether portal in our base, prepared all my best equipment and stepped into it. Then I appeared in the nether and realised I forgot something, I returned to our base fine but when I went back to the nether again there was no portal for me to return. I didn't bring any obsidian so I put all my stuff in a chest and prepared to die so I could respawn back home... But I thought I'd give logging out a try first. When I logged back in the chest was gone with all my diamond stuff  

Then I thought perhaps we should try rebooting the whole system so we both logged out and restarted the console... When we logged back in our game hadn't saved since ~1 hour into our session so we lost everything.