Two things:
1) As so many have mentioned, shooting at a very low f-stop (aperture) will create a very narrow depth of field. This will be compounded by focus point selection (more on that in a moment). Sometimes you WANT a very shallow depth of field, but that really only works if you're sure the camera has locked focus on the part you really want in tack-sharp focus.
2) You have the ability to force the camera to lock focus at specific positions in the image. The 7D has a 19 point AF system and several focus modes. In many of these modes, the camera has some flexibility to lock focus on a target. If the camera is offered some flexibility over what it picks, then it will pick the target with the CLOSEST focusing distance to the camera. That means if you have a shallow depth of field, and you WANT the camera to lock focus on the subject's eye, but there's a something closer and another AF point is positioned over that "closer" object, then the closer object wins. Due to the nuances of the way "phase detect" auto-focus sensors work, the camera does not have to test and evaluate focus by doing a focus hunt through all the points. The direction of the phase shift tells the camera whether the focus needs to come in or go out to lock focus and the amount of phase shift tells the camera how far it needs to move. The camera knows instantly which focus point can lock focus on the nearest element and moves the lens to lock at that position. It's very fast (you paid good money for it to work that way.)
You can read a bit about the focus system here:
Canon DLC: Article: EOS 7D: Sophisticated, Customizable AF System
But there are many books and extensive articles that explain all the nuances of the system. It's actually an extremely good system. It was designed to offer a very high degree of control to action photographers who knew what they wanted. But this means you have some homework to read about the system and learn to control it.
Minimally you'll want to learn to put the camera into spot AF mode. Once the camera is in this mode, it will only use a single AF point to lock focus (the rest of the points are ignored.) You can press the focus select button (upper-right on the back of the body) then use the joystick to navigate the selected focus point around. Put the point directly on your subject's eye, lock focus, and take the shot. This should put the tack-sharp focus directly on that specific point.
I normally return my camera to allow it to auto-select the focus point before powering off (I never know if I'll want to grab the camera in a hurry.) But usually when shooting a subject, I don't use the auto-select mode... I pick the focus points or at least pick the focus zone.
Sometimes a lens misses focus even when the camera would have nailed it. Lenses can have focus error in either the back-focus or front-focus direction. Normally you wont notice a lens with focus error when shooting at even moderate f-stops because the amount of error is usually much smaller than the depth-of-field area. That means even if the lens misses focus, the image still looks pretty sharp and you are none-the-wider to it. BUT... when shooting at very low focal ratios, this can be a real problem. It's possible to test a lens for focus accuracy using a decent focus target (there commercial targets available and also targets you can download, print on your home printer, fold up, and use them to test focus accuracy.) If you find your lens is consistently missing focus (these tests must be carefully conducted to ensure that it is the lenses fault that focus was missed and not the fault of the camera operator. That means the camera MUST be on a tripod and incapable of moving or the test isn't valid.) The 7D has AF calibration which is per-lens (I think it can "remember" the calibration of up to 50 unique lenses that you own. Far more than you'd ever likely need.) If the camera knows that a certain lenses consistently back-focuses by a few focus units, it can be programmed to "lie" to the lens ... telling it to focus just a fraction closer than it really wants. That way when the lens back-focuses, it actually nails the focus right where you want it. That little feature is another reason why you paid more for a 7D.
There is one more thing that I notice some people do.
In the default focus mode (aka "One Shot" mode), the camera wont focus until you press a button to activate focus. This can be a half-press to the shutter button OR you can use another button (you can program back-buttons to start focus.) But here's the important part about "One Shot" mode: Once the camera locks focus... it shuts off the AF system. That's it. It's locked. If the subject moves or if YOU move, the camera will NOT adjust the focus. You'd have to release the shutter button and do another half-press to get it to turn the AF system back on. "One Shot" mode is made for shots when the subject isn't moving. It's focus is more accurate BUT... you must NOT move. I have seen people set very shallow depth of field, focus... and then LEAN forward or backward (which of course means the subject is no longer at that same distance where the focus was locked.)
The camera does have a mode when the focus distance is constantly changing. In "AI Servo" mode the focus system never stops working. It's always checking to see if the focus distance changed and if focus can be improved. You'd think that you should just leave the camera in this mode all the time... but that would be a mistake. Only use "AI Servo" when you KNOW the focus distance will constantly be changing (e.g. action shots). If you know the subject is stationary then use "One Shot" mode. This is because in "AI Servo", the camera believes the subject SHOULD be moving... so even after focus locked, the camera will still try to tweak focus to see if it's changed. Another nuance of "AI Servo" mode is that it's also a "release priority" mode -- that means the camera WILL take the picture when you fully press the shutter button ... whether the shot was in focus or not. And THAT means that if you fully press the shutter at a moment when the camera was testing for movement, you can get an out-of-focus result. In "One Shot" mode the camera is in "Focus Priority". It will NOT take the shot unless it was able to lock focus. Of course it only locks focus once and then it's clear to take the shot (even if the subject moved.)
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