This actually happened to me!!
It was my first deer hunt. I had a Muley at about 50 yards broadside. My heart was pounding so hard the locals thought the Army was doing above ground nuclear testing. My rifle suddenly weighed 35 pounds and it had a small propeller attached to it making it impossible to control (it was going all over the place). The deer kept changing sizes; first it was the size of an elephant but every time the scope got on it it shrunk to the size of an ant. When the earth finally stopped bouncing and I gained control of my faculties, I steadied the cross-hairs on its shoulder and squeezed...oops, forgot the safety. Now I'm ready; took a breath and squeezed again. The bazooka went off! The smoke cleared and the deer was still standing there; a bit confused but still standing. Once again, breathe and squeeze. Bang! Nothing...hmmm OK third time's a charm. Pow!...What the???
Now I'm in a panic. Just how long was this deer going to wait for me to get it right? What should I do? OK, one shot left. Its in the chamber, the safety is off, the deer is in the cross-hairs, the scope is loose and wiggles....What?!!! So, with the thumb of my off-hand holding the scope steady (and upright I might add) I slowly squeeze...pop.
Success! My first deer had only one hole in it, the last shot got him. That was 25 years ago and I was 26 at the time. The rifle was a Savage chambered in .308. The memory...priceless.
If there's a moral to this story I guess it would be, "Don't ever give up." It also helps to make sure everything is in working order.