I very nearly did this one night after the third sector. I was inside the aircraft which had just come out of the hangar. I was going through my checks when somebody came into the flight deck with a problem with a cargo door. After resolving that, on with the show. Been outside so walkaround done. First item on the before start checks gave me a very uneasy feeling. Walkaround not done. I coughed up, got the stairs back and removed all 3 gear pins. Beers on me but it showed me just how easy it is. Checklists are a very good idea.