I shot a buck this year that had no noticeable hole. This was on a drive with a moving deer, the second shot I saw the deer react and slow down a bit, the third shot his back end dropped, I thought I hit the spine, then missed with another shot. He went down, head came back up and then down for the count. When I walked up to him my third shot was low and took out the back knees, not the spine, (shotgun zone, smoothbore, running). I opened up the deer to gut it and the cavity was full of blood but we could not find a single hole in the hide, did not make any sense. It was not until 4 days later when we skinned him out that I noticed a 1/4in slit between the ribs, seems like I hit a tree and blasted either a chunk of branch or lead shrapnel into the chest, that is what put it down.
No the gun was not off, a couple days later I smoked a doe exactly where I aimed.
Weird things happen