I once built a real beaut of a Duna lander. It could crew three, and was driven by two nuclear rockets. It could land on Duna without parachutes and had enough dV left to SSTO and return to Kerbin. I had just recently discovered and started using mods, in particular the nice streamline fairings from KW. Now to fit those long nuclear rockets inside the fairing they would have to go under the lander, but to do a stable landing, they had to go on the sides. Infernal Robotics provided some nice hinges that allowed those rockets to stow under the lander for launching and fold out to the sides for landing. It was simply stunning to see the launch and deployment of this lander - ballet in action. So when Jeb, Bill and Bob arrived at Duna - some two years after launch - those nuclear rockets wouldn't fire. I'd forgotten the #@@#! fuel hose! The poor crew had to circle Duna for another year before they could return to Kerbin. I immediately added and tested the offending fuel hoses, and also made a few other minor changes. I immediately sent Jeb, Bill and Bob on their way again. So, they arrive back at Duna. They fire up the rockets, and - NOTHING! Those minor changes ended up cutting off the fuel hoses during the complicated unfolding of the rockets. I eventually did get a crew to Duna and back using this lander, but it wasn't Jeb, Bill and Bob. In fact, the crew that finally made the landing passed Jeb and co. returning to Kerbin on the next launch window to Duna. I still feel guilty for making Jeb and friends sit in a capsule for years and never actually getting them to Duna; but at least I got them home alive.