What I did:
1. I put a bed in the celing in my stronghold, and I slept.
2. When I awoke, I glitched through the bed and landed on the floor.
3. I entered the End.
4. When I exited the end, I spawned at spawn and the game said my bed was gone.
5. When I got to the stronghold, the bed was still there.

How to reproduce:
1. Put a bed where there is no valid spawnpoint.
2. Die or go to the end.
3. Respawn.

I also got this in 15w50a but it is no longer on the list.

My suggestion: add a new message: "There is no valid spawn location"