In the late 1960s, Klass quietly abandoned his plasma theory and afterwards argued that all UFO sightings could be explained as misidentification of normal phenomena (such as clouds, stars, comets, or airplanes) and/or as hoaxes. Jerome Clark contends that Klass argued in favor of hoaxes more than almost any other UFO skeptic, but that Klass rarely had evidence in favor of his accusations; this position was echoed by Don Ecker, who asserted that during a 1992 debate, Klass made unsubstantiated charges of "drug smuggling" against Australian pilot Frederick Valentich, who disappeared in 1978 after claiming a strange UFO was flying near his airplane.