That horrible Amtrak accident, with excessive speed, was almost certainly caused by human error. While I haven’t seen the cause of the Atlanta Airport fire, placing the backup system activation so close to the primary—thereby enabling one event to wreck both of them—was clearly human error.
Most of the problems you’ll find in you clients involve human error, either in judgment or performance (attitude or skills). Stop redesigning processes that already work well, and focus on people who are making the decisions and doing the work.
Most aircraft crashes are caused by human error. VW, Enron, Exxon Valdez, Wells Fargo—human error. Start with the obvious.