The special-effects technician who ignited the explosives that led to the deaths of actor Vic Morrow and two child actors will testify that misfirings of such explosives are "not unusual," according to the prosecutor in the "Twilight Zone" involuntary manslaughter trial. James Camomile, expected to be a critical witness in the trial of director John Landis and four associates, was due to testify this week but has been rescheduled to testify Monday because of delays in the ongoing trial. Deputy Dist. Atty. Lea Purwin D'Agostino contends that the defendants acted negligently by placing the three actors in the vicinity of the explosives and a low-flying helicopter, which was struck by the explosives and spun out-of-control into the actors.