This is the fourth time this diversity action has been before this Court on appeal. This case has been tried before a jury on three different occasions: in June 1974, in January 1977, and in June 1977. Two heavy tank trucks sideswiped each other. The truck, owned by Dixie lost its left front tire at impact, veered off the road to the left, and overturned, killing its driver, Conway. P's widow, sons, and Dixie Transport brought this action against D, owner of the eastbound truck involved in the collision. Ps claim that D negligently crossed over the centerline of the highway, striking the oncoming truck driven by P. The only living eyewitness was D's driver, John Johnson, who testified that P suddenly turned onto D's side of the road when the vehicles were about a truck-length apart, with a closing speed of 100 mph. Expert witness testimony centered on the various marks made by the trucks at the site of the collision. At the second trial, D did not call its prior expert witnesses used in the first trial. It used Hay who had not been designated an expert witness and who sat at D’s table as the representative of D. When Hay was called to the stand P objected that Hay was a surprise witness. Hay stated that the marks were not made by the D truck but were made when a vehicle of a different type later tracked asphalt from the asphalt spill left by P's overturned truck. He also testified that what he claimed to be D tire marks were not skid or brake marks; he asserted they would have been lighter than the westbound tire mark; that any eastbound tiremarks made by D's truck would have been a lug (or horizontal) type marking, while the eastbound marks, he claimed for D, were made by circumferential tires. Hay's analysis allowed the jury to determine that both vehicles came so close to the centerline that they clipped mirrors as the cabs of the tractors passed on the highway, causing P to lose control of his vehicle. D got the verdict at the second trial. P filed a motion to set aside the verdict of the jury and to grant a new trial. The district court granted the motion for a new trial. The court did not address P's claim regarding a surprise witness. The third jury trial was in June 1977. P got the verdict. The Appeals Court determined that the trial court erred in ordering the third trial because the jury's answers in the second trial supported a judgment for D. The case was reversed and remanded with instructions that the trial court enter judgment for D. P filed a motion for a new trial, reasserting the ground of a surprise witness. The trial court granted the motion for a new trial. D appealed.