I know how you feel. I lost one last year. 35ish yards standing broadside shot with a muzzleloader. Aimed right behind the shoulder, hit her in the right rear kneecap. Tracked her for about 150 or so yards before running out of blood. I have no explanation other than I may have hit a branch or something. I went back and shot it at a target and it hit within a couple inches of where I was aiming. I felt terrible, I hate doing that. I imagine the coyotes drug her down cause the leg was just flopping. I reloaded as fast as I could but never could get a second shot before she was gone. I'd like to say it won't happen again but, as a hunter, I know it will eventually.