Why we need test first development

Here is an other classic example. I bet it will be used in some book sooner or later, like Test Driven Development. Here is the story:
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"The Chinook helicopters could be a risk to fly in cloudy weather because the software which enables them to do this cannot be properly tested.
Fixing the problem will cost an estimated �£127m and the Chinooks will remain grounded until at least 2007. "Read more...