I remember editing and translating a health advisory on the spread of the highly contagious norovirus (I get all the plum jobs), which contained the line 'you should wash your hands after using the bathroom'.
'Surely that's common sense, not to mention common decency', I thought, but if it was, there'd be no need to write it. Perhaps I'm being an arrogant git by thinking it's common sense.
Let's face it, common sense depends on lots of things: upbringing, culture, customs, education and more. It might be better to think of it as common understanding and, as with all understanding, it takes time to become common.
The other thing to remember is that common doesn't mean ubiquitous. Just because most people you know seem to know something, it doesn't mean everybody will.
So the next time you want to whack somebody on the head while shouting 'DUH! IT'S COMMON SENSE!', remember that the thing with common sense, like common knowledge, is that it's not as common as you might think.