Slightly off thread I was taking off in a PA 28 140 with 3 up 1 being an instructor and we selected 2 stages of flap to practice a SF take off, we got off the ground then suddenly found our selves back on the ground for no apparent reason(luckly we were at White Waltham so 1000m to play with) after finally getting airbourne on the climb out we discovered only 1 stage of flap set. As far as we can worh out the last bump on the runway on the first lift off cause the flaps to lose on stage of setting, on a shorter runway we would have been in trouble.