Canon 50mm f/1.4 USM Focus Challenge

The DOF is really thin at f/1.4 and small movement of the model or the photographer can cause misfocus. AI-Servo AF mode might help.

Focus & recompose is another possible explanation, especially if the shift is always in the same direction.

It is my understanding that the actual focusing sensors are bigger than the red rectangles in viewfinder indicate and if a contrasty item is placed near the red border, the camera can focus on it rather than on whatever you placed in the middle of the rectangle.