Gee, so many to choose from....
Early in my PPL training, practicing short-field takeoffs. At a towered airport, busy saturday afternoon, cleared for takeoff, responded appropriately, then talked with my instructor through the steps (flaps, brakes, power setting, etc) for a short field takeoff... not realizing I was still transmitting......
Another; early in the commercial training, I'm taxiing out with my instructor, we come to a point where we need to make a turn on the taxiway - my brain was obviously thinking, since we're on the ground, we can turn by turning the yoke like a steering wheel..... the instructor looks at me, asks with a laugh, "what the he!! was that?"; we laughed, but i don't think I've lived that one down fully yet...
And, after a long flight and a night landing, Grant and I land at one of the more remote airports so i can pick up the car, and drive it back to our home airport. Having taxied so well in the plane after my flying leg, I continue the excellence by deftly exiting the airport's parking lot in the car.... staying squarely on the yellow line all the way...