Have you ever met someone at work or through friends who is really easy going, friendly and generally keeps a middle of the road profile? What do you think their hobbies are? I used to work with Josh who has since moved to Perth. He was a test engineer and we had occasion to collaborate on some products together. One day I found out Josh enjoyed Thai Kickboxing and Brazilian Jiujitsu as his main extracurricular activities. I talked him in to a photo shoot, as I tend to do, and managed to get him sufficiently focussed that I'm glad I never ran my big American yapper on any touchy subjects.
I guess this is just a gentle reminder that the inconspicuous guy or gal at the pub or in the car next to you may just be inclined and fully capable of unleashing a giant can of whoop-ass if given cause. Probably not, but just maybe.