Mistake one was not anticipating the effect of the increased mass. The new payload makes the ship heavier and therefore less aerodynamically stable. It wobbles dramatically, takes more thrust to get into orbit, and causes me to misjudge the throttle. By the time I'm in orbit around Kerbin, two fuel tanks have been spent.
Mistake two was not fixing the flaw that caused the entire ship to fall apart when decoupling the second-to-last stage. I'd tested the new design with some trial runs in Kerbin's atmosphere, but only diagnosed as far as 'I guess it probably won't happen again'. It happens again.
I evacuate Bill from the useless three-man capsule, and use his jetpack to spacewalk over to the one-man moon lander. He boards, and I attempt to detach the two crafts – at which point I realise that the decoupler I used isn't a decoupler. This is mistake three. The two capsules are instead welded together, the larger one stuck to the smaller like a mutated mushroom head. Even with these problems, I'm able to use the second craft's full tank to reach the Mun's orbit. Which is where mistake four happens.
This final mistake is an extension of the third. The original capsule added too much mass to the ship. As the Mun's gravity takes hold, the tiny engine can't reduce the descent speed. It crashes into the surface, killing Bill and his two subordinate crew members, both of whom were strapped dutifully into the first capsule.
Of my three chosen Kerbals, Bill is dead and Jeb still orbits merrily around the Mun. Bob is my last hope. His ship is a slight modification of Bill's, tweaked in the hope of fixing the maddeningly undiagnosable glitch that was causing its predecessors' staging malfunction.
For once, the launch goes well. This ship is no better designed but, after numerous tests and failures, I've learned to pilot through its eccentricities. My second tank finally runs dry during the transfer to the Mun's orbit. Now to see if my modifications work. They do! The ship doesn't fall apart. It does wobble alarmingly, and I finally realise the problem. The charge of the decoupler was too powerful for the smaller remaining pieces, so it instantly ripped them apart. This time, the even tinier pieces shake and strain but remained attached; held together for reasons known only to God and alpha programming.
Reaching the Mun's orbit, I evacuate Bob from the ship, and pilot him towards the second craft. The two halves detach, and the smaller lander begins its retrograde burn to the surface. It's easier without a giant capsule welded to its roof, and the lander slows to a safe speed, eventually – nervously – reaching the ground. Bob Kerbal becomes the first of his kind to traverse the surface of the Mun (as anything other than a puddle, that is). I haven't got the heart to tell him that he'll be there for a while. I left the parachutes on the other half of the ship.