Flat rectangular profiles of the aluminium alloy AA6061-T6 were friction stir welded to butt joints. Welding parameters outside the process window generated a random mixture of good welds and welds with small voids. These voids reduced the tensile strength by 10%, the ductility by half, and the fatigue strength by 20%. Voids can easily be detected on fracture surfaces. Void-free specimens have 10% lower fatigue strength if the fatigue crack extends over a corner of the specimen cross section.