Via Google, I found an informational piece about broken jaws. It told me,
Because the most common causes of jaw fractures are the result of motor vehicle accidents and assaults, the best prevention is to drive carefully and choose your friends wisely. A more realistic step that can be taken is wearing protective devices in sporting activities.
If only I had chosen my friends more carefully. . . Seriously, though, are a large proportion of assaults between friends? This is definitely news to me.
Shock and Jaw 
re: the danger of assault
This reminds me of the saying in a recently-opened fortune cookie from a Chinese restaurant in Flushing, NY:
There is not substitute for good manners, except, maybe fast reflexes.