One of my least-favorite expressions is "that's common sense," because people throw it around when talking about something with which they're familiar and use it to put down others who lack knowledge of same.
Knowing how to operate ANY piece of machinery, for instance, can't be common sense because it requires some type of knowledge that a person who grew up without the technology wouldn't have. Same could be said for fixing said machinery -- things like changing your oil, replacing your headlight, and even pumping your own gas are only commonsensical when you've done it, or seen it done.
That having been said, there are times that the 'common sense' label ought to apply. I saw two such examples today, and feel inspired to write about them because a) not only was common sense violated, but b) the violator -- rather than offer an excuse or apology -- didn't seem to realize what had happened:
(1) In a room with three or more people in it, a speaker should address the person for which something is intended before saying it.
I think we've all seen this applied, or mis-applied. If you work in an office with eight people, there are always going to be two-way conversations, multi-way conversations, sidebars, phone calls, drop-ins, etc. Lots of talking is going on. Unless you signal the person you're addressing with "Hey, [insert name]" and wait for a "Yeah...?" before speaking, the recipient has no reliable indicator that he is being addressed in the first place. So you may go on with some ten-minute explanation of what needs to happen that day, or a diatribe about why a procedure is messed up, or whatever it is you're prattling on about, but all the "recipient" is hearing is broadband white noise. The same type of thing could happen in a crowded car, a lunch table, or anywhere else. The key thing here is that the burden should fall on the speaker. Because that could cut across any divide of culture, technology, and time (unlike, say, how to connect your laptop to your modem) I think it could fairly be put under the 'common sense' umbrella). Bottom Line: If you're addressing someone and he or she doesn't know it, the fault is yours for not 'signaling' first.
(2) Before entering an elevator, people should wait for those who need to get off to do so first.
I think this one pretty much speaks for itself, so there's really no need to belabor it. It's not an arbitrary thing to say off-loaders first, then on-loaders; from a practicality standpoint, it wins, hands-down. Of course, the same could be said for buses, trains, etc. The funny thing is, I'm staying at a hotel in Arlington right now (conference in DC this week), and I just saw this violated twice in a row. The second time, the woman who barged into the elevator as I was trying to exit in the lobby harrumphed me with a drawn-out "Excuuuse me," and I turned to her and calmly replied, "Yes, excuse you."
If you don't know the clutch from the gas from the brake, you're okay. Jared Diamond even has your back -- he wrote an entire chapter on the way people use relative knowledge to (wrongly) make the leap towards cultural superiority in Guns, Germs, and Steel. If someone accuses you of lacking common sense, just coolly reply with an inquiry about whether the person feels that any Maori tribseman (assume none has ever driven a standard) has common sense either.
But if you don't signal out the recipients of your speech in a multi-party environment, or if you barge into elevators without letting off-loaders move first and don't see the problem, I'm a little more inclined to worry.