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Spoilers for Season 8. Follows an alternative ending to season 8.
Sam, Dean and Cas had successfully closed the gates of Heaven and Hell. Pretty soon Sam was somehow brought back to life again (after all neither of the Winchester brothers seemed capable of staying dead). All was going well until they found out that they couldn't get rid of ghosts anymore.
"I don't get it," Dean said. "We burned the corpse sand everything belonging to the ghosts. Why aren't the spirits moving on?"
Sam thought about it and then he realized the problem and felt like an idiot. "We closed the gates to Heaven and Hell where are the spirits supposed to go?"
"We really didn't think closing Heaven and Hell though," Cas said.
Maybe the souls would have gone to Purgatory. But I think that's unlikely as Purgatory is only for monsters. Or maybe closing the gates only affects the angels and demons and ghosts could still move on somehow. But I still think closing the gates would have caused problems.