The image is a body free-falling with two forces acting on it. If we take moments about A, we clearly get anticlockwise moments, and that is what will happen if this were to take place in real life.
However, if you take moments about B, the resultant moments would be clockwise - this is clearly wrong, but why is it wrong? I can see some people saying we have to take moments about the centre of mass, but why is that the case?
-----edit sorry the two forces should act from the same side
------edit diagram edited