DozyWannabee, syseng68k was describing something close to what is done with the shuttle's code. The people writing the software are looking over each other's shoulders looking for faults. The attitude is high fives for everybody if somebody finds a bug, even the person who created the bug. It's one less problem the shuttle will face.
This process is extraordinarily difficult and costly. I suspect what goes on is a subset of the shuttle's process.
And I note a software bug is a bug. A design bug is a bug. All bugs are not one of them. And designers will split the design phase into smaller pieces to decide where the bug happened. All of them will then try to make sure the process makes those bugs harder to find.
Of course, if the customer specifies it wrong and won't budge.... It ain't us software folks, at least. But we still feel line poo when people die.