For me, it'd have to be my manned mission to Mars in SSRSS. Like in the NASA proposal, I sent a lander ahead unmanned to take the crew back into space. Instead of seperate launches for a habitat, rover, and descent vehicle, however, I decided to roll them all up into one vehicle. The MAV was landed near Olympus Mons with the intent of driving up it a short distance and taking surface samples and whatnot. When I got there, though, I had a last second fit of madness and changed my plan. Since the Hab was mobile, I dropped it straight in the caldera instead and drove all the way down the mountain back to the MAV.
KSP - Ares II