I see this done often and I still do it myself from time to time. When one tilts their camera upwards to focus on the eyes of the model then tilts the camera back down to compose the image. Seems harmless, and it may be harmless if you are shooting at f/8'ish to f/16 or more.
(remember shooting at apertures of around f/8 - f/16 and more results in a wide depth of field whereas shooting from f/5.6 - f/1.8 results in a shallow depth of field...click here for a visual example)
Now let's take a look at what happens if we are shooting between f/1.8 to f/5.6'ish.
Imagine holding the camera in this diagram that I
Now that we have tilted our camera, notice where the orange line of focus on our subject has moved to. It's not focused on the eyes anymore. Now our focus has moved several inches behind the eyes. Remember those few inches we had? Now the eyes are out of focus.
In the image of Tiffany above I did this very thing. Here is a close up of the focusing for you can see this error in action. CLICK on this image to load a bigger/clearer version!