The bug
Breaking and placing a claimed bed in the same place causes a second villager to claim it, even though the original owner had not forgotten his bed location. This causes one villager to sleep at night while the other tries to claim the same bed.

What I expected to happen was...:
When replacing the claimed bed, the original owner would either discard his claim or the second villager would not try to claim it.

What actually happened was...:
Two villagers claimed the same bed.

To reproduce:

1. Make a trench two blocks deep
2. Place a bed and a villager
3. Wait for the villager to claim the bed (he will emit green particles)
4. Spawn another villager in the trench
5. Break and place the bed in the same position
6. Check if the second villager claims the bed
7. If it does, use this command:

   /time set night

8. You will see one villager sleeping in the bed, and the other constantly reclaiming the bed