Players standing on or above platforms of solid blocks at Y=127 will be placed several hundred meters above the world spawn point, and sent a client message "Your home bed was missing or obstructed".
In the Nether, the player will be placed at a location, still in the Nether, that correlates (based on 5 Nethers) with the overworld spawn point.

Confirmed in Creative and Survival mode. LAN client behavior not tested.