Timothy Diehl (plaintiff) was a construction worker for IA Construction, Inc. (IA). Diehl was injured on the job while using a road widener manufactured by Blaw-Knox (defendant). Diehl was walking behind the widener, as was common practice, when the widener went into reverse and crushed Diehl’s leg with one of its tires. Diehl brought a design-defect suit against Blaw-Knox. Diehl sought to introduce evidence that, shortly after Diehl’s injury, IA (1) welded a rear bumper on the widener, covering the rear tires; (2) moved the back-up alarm from the front of the widener to the back; and (3) added a warning sign to the back of the widener. The district court declined to admit the evidence, citing Federal Rule of Evidence (FRE) 403 and potential juror confusion, as well as FRE 407. The district court ruled in favor of Blaw-Knox. Diehl appealed.