The defect, in this case, would be in the design. Like a car that would require the emergency brake to be released before starting, but still would start in drive or reverse. Starting it in gear would still be the operators fault, but the requirement for releasing the brake would amplify any negative results of doing so and be a contributory factor in any accidents. Had the saftey not been required to be off before unloading a loaded gun it would be entirely the operators fault.