Dumb luck:
-The top hinge of my door in the 172 broke while doing some commercial maneuvers. I was waiting for the other one to go, and the whole thing to whack the stabilizer. When I opened it on the ground, I had to lift it out of its frame, lest the whole thing would have Tommy Boy-ed on me.
-My first alternator failure happened when I was being observed by a higher up. No biggie, day VFR off the departure end, but still... dumb luck.
Poor judgement: In my first month of instructing, I let let student fly an approach way too fast, landed very far down, squealing brakes, barely made the hold short. Yes, I was an idiot. That one really put me in my place.
I've been fortunate. Dilligent, too, for the most part, but fortunate.