Pope Thinks about Limbo
As silly as this comes across, according to the Bible, you can't get into heaven if you have unrepented sins or if you haven't been baptized. In other words, under fundamentalist mentality, if a baptized serial killer, who later goes to confession, murders an unbaptized victim, the killer goes up, the victim down.
In one of the rare sensible acts in the history of organized religion, the Church dared to think, "Well, that doesn't make any sense."
Purgatory makes sense (although Hell really doesn't) and has Biblical foundation (in the Catholic version; Luther took out the books with references to it).
Nothing in the Bible even hints towards Limbo but the idea of a completely sinless person being condemned to Hell (even if he died on the way to the church) is a little too much for anyone slightly more rational than a Southern Baptist. Eternal punishment for a blameless soul goes beyond "God working in mysterious ways"; that's the work of a complete prick.
Dante's First Circle of Hell was filled with virtuous pagans who were given eternal peace and intellectual fulfillment. Dante commented that he thought that many of them assumed they were in Paradise. That stretches the concept of damnation but it makes far more sense than torturing someone for living in the wrong place in the wrong time.
Ultimately, it's not Limbo or Purgatory that I think is such an outlandish belief but Hell. The Pope did say in Crossing the Threshhold of Truth that while the Church believes in the existence of Hell, there is no way of knowing if anyone, even Judas, has or ever will be sent there. As far-fetched as that may be, I think it's the closest position to reasonable as you'll ever make the concept of Hell.