I forgot to turn on the fuel when I was just going to taxi to, of all places, the fuel pump. It got me through the very brief after start checks (I was only taxiing after all) and from the middle of the apron to the edge of the runway. There I stopped while everyone watching had a bloody good laugh and I realised that there's no way a PA28 would run without fuel long enough to get anywhere near take off.
Two near statistic experiences, though, one as pilot and one as a passenger. The first was ending up in fog in the circuit on my third ever solo flight. The moral of that tale is never trust sea fog to stay by the sea, especially not if you get a mid afternoon sea breeze. I turned away as soon as I realized what was happening but the fog came over me in the turn. Fortunately I'd just done my first instrument flying lesson.
Second was on take off, flying right seat with another PPL. He had made his departure call and as he opened the throttle a microlight called 'short final'. We're rolling and I just catch something move over my right shoulder, yell 'Stop!' which we do, fortunately keeping propellor off the ground, while the microlight lands - in front of a moving aircraft! If I hadn't been there....