No matter how good something looks on the CAD screen, thorough testing will often find problems. Had they launched the MER probes without testing the parachutes, odds are both vehicles would've splattered on the surface. The same applies to the airbags.

I've seen how satellite antennas are tested in anechoic chambers. There's a lot of good engineering that takes place before the prototype antenna is made, but you still have to put it in a chamber and tweak it to get the best performance. Antenna design is still something of an art, although the scientific tools get better all the time.

From the Nova program, I think I heard them say that the equivalent airspeed for a Mach 2 deployment on Mars was 71 MPH on Earth. It's sad that they've closed the wind tunnel, but I can think of ways to achieve the same test results. From recent articles about Burt Rutan's SpaceShipOne, when they encountered a stability problem, they mounted the tail on a truck and drove it across the desert at high speed to verify the fix.