If you build an end portal on the world spawn point, or /spawnpoint on an end portal, you'll be trapped in the end every time you die. This doesn't happen when you build an end portal around your bed, only on your primary or command-set spawn point. Dying in the end with that condition can respawn you to an end with an overworld sky. Reloading the world may reset the background to the end sky.
WARNING: If you accidentally put an end portal on the world spawn point and enter the end, you can't get your world back. Even in multiplayer, all players who join would instantly teleport to the end. Another player can't just remove the end portal.