You slowly lift a bowling ball from the floor and put it on a table. Two forces act on the ball: its weight, of magnitude, mg, and your upward force, also of magnitude mg. These two forces add to zero so that it would seem that no work is done. On the other hand, you know that you have done some work. What is wrong?

You slowly lift a bowling ball from the floor. One force is acting on the ball due to its weight in the downward direction (mg) and another external force acting on the ball whose magnitude is also mg in the upward direction. These two forces balance each other and gives there is no net force acting on the bowling ball which result there is no work is done. This signifies that you move the ball with constant velocity against the gravity. Since acceleration is the rate of change of velocity, therefore the net acceleration on the ball will be zero and that results the net work done is zero.