1. The wind did it.
Although it's absolutely true that the wind can and does alter the flight of a bullet, in general the phenomenon is most apparent over long ranges. However, that doesn't mean that it's impossible that the wind caused you to miss a bullseye 10 yards away with your .45-caliber 1911. There could very well have been a freak mini-tornado that occurred in a very localized manner right in between the muzzle of your gun and the target 10 yards away. Challenge your friends to prove that it didn't. They can't, can they? This is a good time to gently point out that they should be thanking you for having brought this potentially hazardous weather development to their attention so it wouldn't interfere with their accuracy.
2. These sights are off.
If you're noticing that the bullet impacts are to the left of where you were aiming, you'll want to move the rear sight right. If those impacts are to the right of where you're aiming, the rear sight needs to move left. If those impacts are all over the target in relation to where you're aiming, it's obvious that what's going on here is that your rear sight has achieved sentience, cranked up the stereo, invited a bunch of friends over and is dancing the Macarena. It's nothing to worry about; sooner or later that rear sight will tire itself out and you'll be able to go back to hitting the bullseye with perfect precision. Probably when your friends aren't watching, but that's just how it goes.
3. Stupid handloads my buddy made for me.
There are a lot of wonderful things about handloading, also known as reloading. It saves money, it allows you to tailor your ammunition for specific tasks and guns, and it helps conserve resources. However, for the purposes of this list, the best part is that many casual shooters don't do it and therefore don't know what can and can't go wrong with handloads. So if your target looks like an unholy collaboration between Jackson Pollock and a bouncy castle, simply blame your ammunition. "My buddy handloaded these for me," you might say, shaking your head sadly. "Oh, no, he's from Canada. You wouldn't know him."
4. Glitch in the Matrix.
Sometimes you get those sights aligned perfectly, squeeze the trigger with just the right touch, perform a precise follow-through...only to have a bullet hole inexplicably appear somewhere other than where you meant for it to go. It's quite obvious what's happened: a glitch in the Matrix, of course. Perhaps some pesky One realized that he no longer has to obey the laws of the space-time continuum and therefore took the opportunity to move the target around in between your (perfect) shot and the (not-so-perfect) bullet strike. Or perhaps the machines that control our every sensory input just got a bit overheated. If your friends display any incredulity, simply insist on addressing them as "Copper-Top" until they drop the matter.
Oh, those pesky Greys! Whether it's building the Pyramids, sinking Lemuria or bending the laws of physics to make your bullet holes spell out "PHONE HOME" on the target, they're always up to some tomfoolery or other. Unfortunately, many skeptical folks among your friends might not believe that a species mastered interstellar travel for the sole purpose of making it appear as if you missed the 10-ring. That's why you shouldn't ever directly blame the aliens. Instead, every time you miss, say "weather balloon," "swamp gas," "lenticular cloud," "St. Elmo's Fire," "ball lightning" or "sleep paralysis." Just make sure you wink when you do it...and keep the non-winking eye alert for the presence of oddly featureless men dressed all in black.
What's your favorite excuse, ahem, reason for a subpar performance at the range? (Not that you need one, of course.)