The idea that Yuri Gagarin wasn't the first human to travel into space, he was just the first human to return safely from space is haunting.
The concept that the USSR got ahead in the space race by a willingness to sacrifice a number of souls to the perils of space, and went to great lengths to cover it up, still seems plausible in my mind.
Putting humans into space is incredibly dangerous, and there are so many ways for a human to die in space. I've always felt there was a curious lack of death-in-space in the history of the early days of the space race, on both the Soviet and American sides.
Even though it is almost certainly warped by Cold War fear mongering, the idea that the Soviet Union shot capsules out of orbit, frequently had cosmonauts burn up on reentry etc just seems to fill in the gaps nicely.
Anyhow mostly conspiracy theory nuttiness, but I still believe there are nuggets of truth in there somewhere...