That the "armor plate" they are using to protect the battery pack isn't strong enough to deal with some road debris. They seem to have underestimated exactly how much energy they were dealing with. And they could have made that metal thicker but it would have negative affected the range.
It's already much stronger than the typical protection of gasoline tanks -- and it was claimed that it was a very exceptional circumstance that allowed that armor plate to be penetrated.
That the rate of "fires from a collision" in a Tesla is 25x that of the average US car. The OTA update reduces the number of undercarriage collisions and thus the number of fires.
