The worst case of this for me involved a gentleman of advanced years, who had spent most of his retirement savings on a huge floating paddle wheel that generated electrical power. Beyond the apparatus itself, he had bought a crane to move the generator to the river! Based on the best-case efficiency of a well-designed undershot wheel, his design was surely much less than 10% efficient. He wanted me to evaluate his design after he was fully committed financially. He was just sure he was going to make tons of money each month selling power.
How do you tell someone they just blew their retirement savings?