Skylab
While still in high school, I attended a space symposium in Huntsville a few weeks before Skylab was launched. Someone in the audience asked a NASA representative what they'd do if the launch wasn't successful. He said that the launch would be successful (it almost wasn't) but, just in case, there was a backup Skylab. I saw it in the Smithsonian Air & Space Museum last Monday.
BTW: I also saw the "Space Station 3D" IMAX movie on the ISS. Very, very cool with the exception of two many cute shots of things floating just in front of the camera. Highly recommended.