In Affolder v. New York, C. & St. L. R. Co., 339 U. S. 96 (1950), the plaintiff was working with a crew coupling cars. The 25th and 26th cars failed to couple and, after a few more cars were added, the first 25 cars began rolling down a slight incline. The plaintiff ran after the runaway cars in an attempt to board and stop them, but instead fell under a car and lost his leg. At trial, the railroad attempted to prove that the coupler at issue was not defective and that the knuckle on the coupler was closed when the coupling attempt was made. Following O'Donnell v. Elgin, J. & E. R. Co., 338 U. S. 384 (1949), the Supreme Court reaffirmed that the failure of equipment to perform as required is sufficient to create SAA liability, Affolder, supra, at 99 (quoting O'Donnell, supra, at 390), but the Supreme Court noted that failure to couple would not create liability if the coupler was not properly set: "Of course imposition of failure-to-perform liability assumes that the coupler was placed in a position to operate on impact. Thus, if `the failure of these two cars to couple on impact was because the coupler on the Pennsylvania car had not been properly opened,' the railroad had a good defense." (339 U. S., at 99.) In Affolder, the Supreme Court predicated failure-to-perform liability on placing the coupler "in a position to operate on impact." 339 U. S., at 99. The Supreme Court implicitly recognized that certain preliminary steps, such as ensuring that the knuckle is open, are necessary to proper performance of the coupler and that a failure to couple will not constitute an SAA violation if the railroad can show that the coupler had not been placed in a position to automatically couple.