You've driven 420 miles from your last fill-up. Your gas gauge reads 1/8th full. You assume that means you still have 60 miles' worth of gas. You bypass gas stations, as you are in a hurry to reach your destination. The car stalls out two miles from the nearest gas station. You don't make it. What happened? Your assumption was incorrect. Had you ever tested the assumption by driving around with a spare gas can and seeing how far you could go after reaching the 1/8 mark? If not, it is an untested assumption.