I was allowed to open the Gemini lunar flyby program, despite not having opened the Gemini program. I assumed I was developing the tech to send people on a lunar flyby, but when I launched the manned mission I found I had a completely undeveloped Gemini capsule on the end of a very efficient rocket. Presumably, someone at NASA was saying "OMG, we forgot to build the capsule! Knock one together in your shed, quick. If we paint it nicely, no-one will notice." My astronauts noticed as they died.

There were also a couple of occasions where a manned flight failed in orbit, or in translunar space, and didn't come back to Earth, but the crew somehow survived.
And lastly, I was told I needed a Saturn V to get the Viking Lander to Mars - that's not what was used in the real world.