I did that... Shooting on a flat surface everything is okay.. but for example shoot a person and focusing on their nose tip, I get the lips in focus.
In that case I would say the subject is moving slightly before you press the shutter button. Another possibility is this: the AF points are larger than you'd think, the actual AF sensor area is greater than the size of the box you see in the viewfinder, so even though your box may be positioned on the nose, the sensor may actually be covering the lips as well, and since there is far more contrast on the lips than on the tip of a person's nose, the camera would naturally focus on the lips.
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