As you were pushing the box of potatoes, the forces of friction and air resistance were always working against the motion. The work these forces did to the box of potatoes dissipated energy into the environment as heat.
At some point, you were working against gravity, but unlike the work done to overcome friction, the work you did against gravity was stored as gravitational potential energy. That energy was released as the force of gravity was doing work to the box of potatoes as it fell to the ground. Since the height of the box was back to where it originally was at the beginning, the net difference in gravitational potential energy was zero and the net work done by the force of gravity was zero.
If the cliff were not 10 meters high, the total work done by ALL forces would still be zero if the box of potatoes started and ended at rest, but the amount of energy stored as gravitational potential energy would no longer be zero and the net work done by the force of gravity would no longer be zero.
The rage you feel toward your Uncle Bob though - would remain infinite.
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Haha.. that's funny :-)