I've got three quick ones. Most were due to young&stupid syndrome.
1: My dad bought me a shotgun for deer hunting when I was 14. We took it to the range and it seemed to shoot OK at the with slugs. Didn't try buckshot because nobody in my family uses it. I have missed probably a dozen deer with that gun (including a large doe standing broadside at 10 yds) I just knew I couldn't have missed those easy shots so I asked a knowledgeable old man why I kept missing. He told me that the gun my dad picked out for me has a Full choke and was probably bending my slugs out of shape and causing inconsistant flight
2: I was 12. First time bowhunting from a treestand. Shot angle didn't occur to me. I shot 3 arrows at a group of does. Missed every shot. Climbed down and retrieved my arrows. Later that evening the does came through again. Another three strikes
3: About 13. bowhunting on public land during the firearm season. At the time I shot with both of my eyes open and did fairly well this way when shooting 3D targets with my neighbor. I had a doe come up a valley and stop about 20 yds quartering slightly. I came to full draw, found my anchor point and released. my arrow flew left and completely missed the deer. In retrospect I realized that I was aiming with the "Phantom" pin rather than the actual pin.