One of the things that takes place in the dozens of Mormon temples is baptism for the dead. Members of the church in good standing are baptized in the place of deceased individuals who did not receive Mormon baptisms during their lifetimes.
The idea is that these people, in the afterlife, will be able to either accept or reject this baptism. This way, people who didn't have the opportunity to be baptized into Mormonism in life can still get the eternal benefit of baptism after death. This is a big part of why Mormons are often obsessed with genealogy. The more they can uncover about their family history, the more names they can take to the temple for baptism.
But...how do they expect to give everyone the "opportunity" to accept baptism? How do you save the billions upon billions of people from the last few millenia for whom there is no record of any kind? Do you just start guessing names? Or will there be some kind of final blanket baptism for "everybody we missed"?
The idea is kind of cool in theory, but it's laughably impractical. Hundreds of thousands of Mormons are spending hours and hours digging up names and taking them to the temple despite the fact that there is hardly a chance in making a dent in the billions upon billions of the unsaved. And they don't seem to feel like there's something inherently flawed in the logic.
And that doesn't seem right to me.