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.
Sadly, there are far more than 1000 parts in a Shuttle. The good news is that most of then have probabilities with a few more nines after the decimal point.