Here's one of those phenomena--if proven correct--that emerges from complex systems that we don't really understand. A German automotive magazine recently performed extensive testing of crashed vehicles and found that current designs tend to mask critical structural damage that could lead a repaired car to fare very poorly in a subsequent accident.
I don't think it's feasible to mandate a re-crash test for cars, but it's interesting as this evolution is seen in motor racing as well. Modern F1 cars use carbon monocoque chassis which absorb much more energy than a skin-and-skeleton structure. But they do so only on the first impact. Second and third impacts can cause the chassis to rapidly disintegrate if it is not nearly vaporized.
So it goes with road cars as well: safer on the first impact, but potentially crippled thereafter.