Well the three years I have been in the hobby I can think of only one think that I did that was dumb. While mounting the control surface for one of the ailerons on my 90 size 540 T I had started the holes punched through the cover into the wood to mark where the surface control would go. After popping the holes I realized that they were punched on the wing and not the aileron its self. Never mounted the control but came close. The small holes are still visible today.