True story. When I was a young Sgt (3 stripes) stationed at Malmstrom, I had a VW Square Back for which I did a complete engine rebuild at the base hobby shop. After reinstalling the engine (rear engine) and wiring everything back up, I sat in the car, put it in neutral, clutch in (habit) and turn the ket to start position. Nothing happened. As I sat there pondering what the issue might be, with the key still in the "on" position, I began running the shifter through the gears. At one point, for no particular reason, I put the shifter in reverse. As I did, the engine cranked over and sprang to life! Now I was really puzzled.
I pondered the situation for a minute, then realized what I must have done. The wiring for the reverse lights and the solenoid were in close proximity and, yes, I hooked them up wrong. I later found that when I'd turn the key to the start position, the reverse lights would come on. It was an easy fix, but boy did I feel stupid.