Vladimir Komarov
There were mechanical issues that prevented the Soviets from sending a man to the moon first and some other tragedies along the way. Vladimir Komarov had a terrible malfunction that caused his death in 1967. Now, there was a black mark because someone had died.
One Small Step
The Americans were also taking charge and fixing their issues, which made things worse for the Soviets. Finally, Neil Armstrong and Buzz Aldrin went to the moon on July 20, 1969, causing the US to win the Space Race and wiping out the initial lead for the Soviets.