This actually happened today and to me.
I have many hours in this game. I try to avoid spoilers and have been pretty successful at that. Still on my first playthrough on my first world (I don't switch worlds and I don't cheat, unless destroying your boat to take it through a portal is cheating). I'm at end-game, looking for the 5th boss. I have played very carefully and fully upgrade my main equipment before taking on the boss. I even keep an Emergency chest of my last equipment for "just in case" scenarios. I got to the point where I can sword and board Lox or half a dozen Fulings. Beserkers aren't that hard after a few arrows. Deathsquitos are a nothing but a nuisance. All other biomes are "comfort zones" for me. You get the picture.
However, I hadn't found any clues where the 5th boss is. I had cleared out at least half a dozen or more Plains and at least a dozen Fuling camps to no avail. I had no idea how to find him. So I did a careful Internet search and found out the stone is harder to notice than the ones in other biomes. I probably missed it. Crap.
Crap. I would have to go back through all the Plains that I had previously cleared out and "leave no stone unturned." Crap. Figured I would just finish clearing out this Plains area before I started that quest. But then, lo and behold by clearing the current Plains I was in I actually bumped into the sacrifice altar for the 5th boss, without ever getting a map indicator where it was. WOW WHAT LUCK!
Carefully backtracking and getting everything ready I sail my ship up with lots of wood and Portal equipment to set up camp nearby. I'm so excited that I sail through the night and make landfall in the dark. Some creatures are nearby, but I already cleared the whole area and I know I can handle it so … so what. Oh, wait, they're right on my as I exit the boat – OK, I'll just dispatch them, like I had done a million tim — OOMPH. Darkness envelopes me.
I awake at my home. Naked, tired, hungry. I didn't have time to set up a Portal. I had been moving my farthest Portal to make it right next to the altar. It's half a world away from my closest Portal. And it's my galleon that's at the shore by my grave, so I will need to build another boat even.
Then sail it.
In the Plains.
With my backup Wolf/Silver equipment.
I should be fine, but Oof, what a mistake for someone who normally plays so careful. I was just so close to the end.
TL:DR I bumped into the altar for the last boss but died without a Portal anywhere nearby. My best equipment and my best boat are there too.