Feasibility study: the questions that change the answer

Most feasibility studies confirm what the person commissioning them already decided. The market section is optimistic, the cost section is the vendor quote, and the sensitivity analysis moves one variable by ten per cent. The questions that change an answer are usually uncomfortable: what is the smallest version of this that still works, who is already serving this customer and why would they switch, and what happens if the main assumption is half as good as assumed. Which question has flipped a decision you were involved in?