Basically when you join in a friend’s world, there is a chance that you will join with nothing at spawn. Even if you’ve spent hours earlier in that world, when you join you could randomly lose everything. 

Stream: https://www.youtube.com/watch?v=U1xQ1YnW0K4 (visual)

Video: https://www.youtube.com/watch?v=auUlcCwebNo (discussed, happen to the same guy twice)