What procedure can inadvertently reverse the direction of rotation of a customer's three-phase motor?

Reversing the rotation of a customer's three-phase motor can occur when any two of the three phase wires are swapped. In a three-phase system, the direction of motor rotation is determined by the phase sequence of the power supply. If two wires are interchanged, it effectively changes the phase sequence, leading to a reversal in motor rotation.

When considering the low side and high side of a transformer bank, both actions—reversing two wires on either side—can result in the motor rotating in the opposite direction. Therefore, both operations can achieve the same end result by altering the phase relationships. This is why the choice indicating both actions is correct; they both will inadvertently cause the reversal of the motor's rotation.

Changing the voltage source does not influence the rotation direction unless it also involves changing the phase connections, which isn't what the question specifies.