It is possible for a bed to become completely obstructed and have no valid respawnable spaces around it, even in cases where the player should conceivably be able to respawn in a crouching or crawling position. The bed will as a result kick the player to world spawn instead of spawning the player sneaking or crawling.

To reproduce, just fill the area one block above a bed with top half slabs, making sure to keep the bottom clear, then attempt to respawn at that bed. It will completely disallow respawning instead of the expected behaviour of respawning in an alternate pose to fit the available space.