Its interesting to hear what others have experienced when they absolutely knew they had a sure kill but something went wrong:

I was hunting in a draw facing away from me dropping from my position to 20-30 feet down to the bottom about 30 yards out. I was leaning against a small pine with couple more small pines just in front of me when a nice buck, 180 lbs or so and rack outside of his ears jumped from a honeysuckle bed in to the draw bottom 30 yards out....I raised my rifle, a 30-30 marlin lever action with 3-9 scope. I placed the cross hair on the buck's shoulder, cocked the rifle, and pulled the trigger....the buck jumped straight up and charged up the draw right at me...when he was ten feet from me I hollered and jumped up scared as crap as he zeroed in on me..at the last moment he turned right and jumped from the side I was on to the other side about 20 feet across...cleared it easily....after I quit shaking I followed the path the buck took for 150 yard or so and finally realized that I must have missed.....I couldn't believe it...I went back to the spot where I was sitting, sat back down and surveyed the scene...well about four feet in front of my location was a small pine with a clear bullet hole in it...I said to myself I didn't see that pine when I shot so I raised the rifle up and sighted at the hole...guess what? At that close distance the scope actually looked around or through the one inch diameter pine...in other words the scope had blurred out the small pine and I had shot it...the bullet must have hit the buck because of the way he jumped but it was more like hitting him with a rock rather than a bullet....ouch!

Some years ago, I had cataracts in my right (shooting) eye and had to switch over to the port side. Shouldn't have been too difficult, as I'm a natural leftie for most things.

Sitting in a pile of rocks, saw a beautiful 4 point buck (western count) come moseying out of the pucker brush with several younger bucks. Downhill shot and about 300 yds. Shouldn't have been difficult with the old Ruger M77 in 7 RM. Pulled a bonehead stunt and aimed for the top of the back to allow for distance. Yup - the shot went right over the back. Cranked another cartridge in and gave a "Texas heart shot" at the fastly departing buck.

My companion and I glumly crossed the canyon to see what happened. Just about across, spotted the outline of a deer in the brush where the buck had exited. Figured that was him and gave another shot. The deer tumbled down into the draw and my buddy raced over and started laughing. Had nailed a fine spike. :(

I was still-hunting through some sparse high-country rocky timber, hunting muleys. A flash in the peripheral vision focused on two bucks exiting at 100-150 yards away. I was shooting a Browning A-bolt 338 Win Mag, with 250 Sierra Gamekings. I swung on the larger buck and pressed the trigger and the buck dropped as if electrocuted. Examination of the scene revealed a 4" diameter spruce took the bullet dead-center before the buck did, but the bullet continued on and broke the neck of the buck. I still have the mushroomed bullet as it basically just dropped out of the far side of the neck of the buck when skinning. A nice 27" 4x4. I got lucky on that one.

I snuck to within about 35 yards of some feeding does near the end of last season. I set the crosshairs on the middle of the neck. I wasn't used to the trigger on my new gun, it was about 8lbs in pull weight, but I wanted to use it. I ended up pulling the shot and blowing the lower jaw off. The doe ran around and began to run towards me, I was still well concealed. It was within 10 feet and I shot it again in the throat, which basically severed the head at that range. I felt terrible about it, because it was an easy shot that I screwed up. I filled two more doe tags that day with the .280 and one shot kills.