If there are 1000 parts for a space shuttle and each of them has a 99.9% chance of working, then the chances for no malfunction at all is only about 37%.

By multiplying .999 by itself 1000 times without rounding, the true result is found, but if each time it had been rounded up a crew might have piloted the spacecraft and met their doom in an explosion.

When manipulating equations, the numbers should also not be plugged in until the end.