If you have the focus setting on AI Servo or AI Focus then as soon as you half press the shutter the camera will still try to keep adjusting focus since it expects the object to keep moving. Therefore if you try manually focus the camera will override this and try and refocus on the object.
You need to switch your focus mode to ONE SHOT. Then if you half press camera will focus only once.. then you can refocus manually without a problem.
I guess you missed his edit from night night at the bottom of his post.








