By far the worst mistake I've made is to take off with the primer unlocked. I was flying the Alarus which can be out-climbed by canal barges in locks at the best of times and this day it was close to +30C. Just out of the ground effect the climb was about 100fpm, but I managed a low level circuit to land on a crossing runway. I don't recommend it.
I do remember pushing it in before start, but I must have not rotated it properly (or rotated it exactly 180/360 degrees). It was a bit easy to pull out on this particular aircraft, so I guess it worked its way out before departure. Remember, primer in AND locked!
Gareth.