Set up fuel lines incorrectly in asparagus staging, staged wrong tanks that were nearly empty. The tanks shot up past the rocket and then ran out of fuel, stopped and then came back down, landing on my capsule and killing my kerbals. *facepalm*
My only real 'A HA! Moment' was when I figured out that I don't need to switch to docking mode to get translation. (Before then I just kept switching between regular movement and docking mode to get rotation as well as translation.)
Working on this right now, I'm using it mainly to train on rendezvous and docking. Might also be used for refuel depot later. EDIT: Docked my first SSTO, was easier than I anticipated.