Nice to see a few replies that are serious contributions.
Tesla has made a few design mistakes, in the midst of many excellent design decisions. Remember the early hoods that were too thin? Now it is looking like the exposed location of the battery pack was not mitigated by enough armor to withstand lever-aided punctures.
The rear wheels are about where the lowest clearance occurs. The front bumper bottom looks to have about the clearance of the forwards part of the battery. That means that the energy storage container is almost guaranteed to take the hit for any object whose height falls between the two clearances.
So far, the armor has handled gouges from trailer hitches--the ones that did not pop up from leveraging. No fires, at least. But the two fires from road debris show that the protection is insufficient. Car handling and occupant safety has remained okay, but the delayed fires are spectacular enough to trigger concern.
I expect that some engineer pointed out the battery vulnerability to road debris during the design phase but the comment got lost. Looking good and underside aerodynamics are good arguments for the approved design, but now it is starting to look like a design mistake. After all, if someone had proposed a flat (but armored) gas tank covering the bottom of the car, the idea would have been shot down quickly. (It's a worse situation, gasoline vapor at the same time as sparks.)
Fortunately, some sort of shield under the front bumper might handle the Model S problem. It is the same principle as an outer defense line out in front of the armored protection for the vulnerable stuff.