The below is a screen grab from the NTSB briefing on the Boston battery failure:
The circle marks the location of an electrical short that they discovered in one of the cells.
The only thing I can conclude from this picture is that this particular short probably resulted from cell over-temperature.
I make this statement because the short is located along the centerline of the electrode plates where the least heat dissipation occurs. It would also seem that any physical manufacturing defect would be found along the edges of the plates.
It should be pointed out that there may also be other shorts that were not pictured and if they exist, they may have a different characteristic than this one.