My most important failure, is the reason why I switched 100% to traditional archery tackle. It was the early 2000's and I was hunting with my compound. I had a nice 8 point about 20 yards from me, broadside, in an open field. I drew, looked through the peep and lined up my pin. Released the arrow, and watch it bury into the ground directly in front of him. Needless to say, that he took off. When I looked at my bow, my sight was way out of position, as I must've hit it on the way in while waling in the dark. I never checked, and this was the turning point for me.
From that moment on, I have never hunted or shot any compound bow ever again. If I miss, it is all on me, not an equipment failure. Just me, the stick, and the string, and that's how I like it.
As was mentioned above, we can always learn from our mistakes, and I learned a valuable lesson that December day.