The Hubble telescope went up in 1990 with the most precisely wrong mirror ever made. It had been ground and polished with great care to exactly the shape a test instrument asked for, and the instrument was out by 1.3 millimetres because a fleck of black paint had worn off the cap of a measuring rod and a laser had bounced off the bare metal underneath. So the edge of the mirror came out about two thousandths of a millimetre too flat, and every picture blurred. The rescue only worked because the mistake was so exact: knowing precisely how wrong the mirror was, they could build it a pair of glasses, and in 1993 astronauts went up and fitted them.