Let's say the player is in the overworld at 800, 64, 800.
They then enter a nether portal
Pressing F3 shows they're at 100, 64, 100 (x/z coordinates divided by 8)
If the nether doesn't load and the F3 screen says "Waiting for chunk" and they either disconnect or get timed out, sometimes when they reconnect, they will be placed at 800, 64, 800 in the nether. This is extremely dangerous as it has often placed me at a height which I've fallen to my death and lost my items from, and it has also placed me above lava. My fate was predictable. I've found that when I fell to my death and teleported asap back to where I died, my items were gone. Far before the 5-minute despawn timer could have elapsed.