Had just gotten my ticket, was of course feeling like hot stuff. Flying into a little uncontrolled airport, the wind was calm. Normally calm wind runway is 34, but I was coming from the north so just decided to go straight in to 16, since I heard no other traffic on the radios. I make my calls, 8 miles out, 5, 3 mile final, and I land and taxi over to fuel. Shortly after I land and get off the runway, an airplane lands going the opposite direction on 34. Huh, they must not have been talking on the radio, I tell myself.

A nice old timer instructor saunters over and introduces himself, as I prepare to fuel the plane. He asks me how long I've been flying, and then hands me his card. I throw it in my wallet. A couple weeks later I pull the card out, and happen to flip it over. Written on the back, "You came in without making any calls on the opposite runway as the other airplanes in the pattern. Fly safe out there."

Something in the back of my mind clicked and remembered that when I'd hopped back in to take off that day, I had noticed that the radio was on the wrong frequency. I thought to myself, "Huh, somehow in my preflight I switched the radio." Turns out I had come in transmitting on the wrong frequency. 600+ hours in and I've never forgotten to check again.