The controller gave a crossing clearance to a tow truck, forgetting he had also cleared an aircraft to land. The work of an air traffic controller is very much dependent on the mental gymnastics of his or her brain. Although forgetting a clearance might seem elementary, the consequences can be devastating! This incident could have ended dramtically, were it not for the fact that the pilot of the approaching aircraft saw the tow crossing the runway and performed a missed approach, or go-around. Another safety barrier in the aviation chain worked as planned.