I was playing it on my own time, just doing a simple Munar landing mission for the fun of it, when I made a fatal mistake. I put about half as much fuel on the lander as it needed. It landed safely on the surface of the Mun, but now doesn't have enough fuel to get back into orbit around the Mun, link up with the return vehicle, and get the Kerbonaut back to Kerbin.
However, I want to stage a daring rescue mission in the form of galactic road-side assistance. I'll launch an unmanned probe to the Mun along with some extra fuel, and then use the probe to refuel the lander for it's return journey. So this should be a relatively easy procedure, provided I don't make any more major fuckups.