Maybe owner held it wrong way?
PS
Got mac notebook at work (for Safari's sake) with exploded battery. Torn apart aluminum cases looks scary.
PPS
Li batteries have built-in controller, that is supposed to prevent such situations (hm, I thought it could only happen when charging). Logically faulty controller should be a pretty common problem, yet I've only heared/seen such things only in connection with company that has
invented rectangular shape with rounded corner.