W. R. Reeves filed suit under the Federal Employers Liability Act against his employer, Central of Georgia Railway Company, seeking damages he allegedly suffered when the train on which he was working derailed near Griffin, Georgia. The liability of the defendant rail road was established at trial, and the issue of damages remained to be fixed. Several physicians testified regarding the injuries received by Reeves. Reeves also testified. On the witness stand, he said that an examining physician had told him that he would be unable to work because of a weakness in his right arm, a dead place on his arm, stiffness in his neck, and nerve trouble in his back. Why did admission of this testimony into evidence constitute reversible error?