You have successfully completed two intensive years of physical and mental conditioning. Youre ready to go into space!
What role would you like to have on this mission?
WRONG!!
Command Module Pilot: Sorry!! You got the German measles and couldnt make the mission, but dont worry, they had a backup available.
Similar to the Apollo 13 mission which was commanded by James A. Lovell with John L. "Jack" Swigert as Command Module Pilot and Fred W. Haise as Lunar Module Pilot. Swigert was a late replacement for the original CM pilot Ken Mattingly, who was grounded by the flight surgeon after exposure to German measles.
Quick, go back and choose another occupation, the rocket seems to leaving without you!