Jeffrey McMahon v. Robert Bosch Tool Corp., 5f4th900

Plaintiff Jeffrey McMahon was injured when the auxiliary handle of a RotoZip Model RZ20 allegedly detached while the saw was operating. He maintained he did not slide the lock pin or depress the handle‑release button before the detachment.
McMahon sued Bosch (manufacturer) and Lowe’s (retailer) for strict products liability (design defect), negligent design, negligent failure to warn, and negligent supply of a dangerous instrumentality.
McMahon retained mechanical engineer Philip Buckley to offer opinions that the handle connection design was defective, that wear could reduce holding force and reduce the two‑factor lock to one, and that the saw should have had an interlock to stop the motor if the handle detached.
Buckley admitted he had not attempted to reproduce a detachment without depressing the release button, and his proposed testimony assumed the release button had been depressed; he also did not develop alternative designs (drawings, prototypes, or literature) or opine on the adequacy of warnings.
The district court excluded Buckley’s testimony under Daubert/Fed. R. Evid. 702 as irrelevant and unreliable and granted defendants summary judgment because expert testimony was necessary; the Eighth Circuit affirmed, holding the exclusion and summary judgment were not an abuse of discretion and that McMahon had waived a res ipsa argument below.

Issue	Plaintiff's Argument	Defendant's Argument	Held
Admissibility of Buckley’s design‑defect opinion (handle detached)	Buckley can show the handle connection was defective and caused the involuntary release.	Buckley’s opinion assumed the release button was pressed and thus does not fit McMahon’s factual account; lacks fit/relevance.	Excluded: opinion lacks fit to the facts because Buckley assumed a materially different event than plaintiff described.
Admissibility of Buckley’s alternative‑design/interlock opinions	Saw should have had an interlock or different handle design; Buckley identifies feasible alternatives.	Buckley failed to show feasibility, prepare designs, test prototypes, or cite supporting literature; opinions are undeveloped.	Excluded: alternative‑design opinions unreliable and speculative.
Necessity of expert testimony for product‑liability claims / res ipsa argument	McMahon: his direct observation of involuntary detachment allows juror inference of defect; res ipsa could permit submission without expert.	Defendants: complex technical issues require expert proof; plaintiff never preserved a res ipsa theory below.	Summary judgment affirmed: expert testimony was necessary; plaintiff waived the res ipsa argument by not raising it in district court.

Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993) (standards for admissibility of expert testimony under Rule 702).
Kumho Tire Co. v. Carmichael, 526 U.S. 137 (1999) (gatekeeping extends to all expert testimony and requires relevance to the facts).
Gen. Elec. Co. v. Joiner, 522 U.S. 136 (1997) (abuse of discretion standard for excluding expert testimony).
Neb. Plastics, Inc. v. Holland Colors Ams., Inc., 408 F.3d 410 (8th Cir. 2005) (expert must consider relevant facts; opinions that fail to fit are unsupported).
Lauzon v. Senco Prods., Inc., 270 F.3d 681 (8th Cir. 2001) (expert testimony must be consistent with the plaintiff’s account and adequately test alternative explanations).
Hickerson v. Pride Mobility Prods. Corp., 470 F.3d 1252 (8th Cir. 2006) (under Missouri law, juries may infer defect via a res ipsa‑type theory in some cases).
Boerner v. Brown & Williamson Tobacco Corp., 260 F.3d 837 (8th Cir. 2001) (plaintiff must show a feasible safer alternative, not merely suggest one, when relying on alternative‑design theory).