Well, if I had a specific example in mind, it would be "fair use". (What started this was talking about the bnetd case brought up in the SCP forum)
Unfortunately I can't think up a case, right off the top of my head, where it was mentioned that one side was unclear about the law - probably because that's not the kind of thing you want to be saying in court.
But this comes from me poking through laws such as the DMCA and Patriot Act and not being able to interpret the entire thing at all. Both are very important; they deal with things like being sent to prison for life without a public trial.
If I were to sit down and just analyze the documents, I doubt I could figure out every small nuance in them.
Now, I could take a career in law, and possibly then I would be able to catch all the little nuances. But, what, are we all supposed to become attorneys just so we know what we can and can't do, and what will happen if we do this or that?
I hear "use common sense and you'll be fine". But then you get into things like filesharing. If I download music by someone, that I'm not going to buy regardless (or especially if I don't listen to it), what am I doing wrong? I'm not depriving anyone of anything. In fact, I may go to my friend and say, "Hey, check out XXXX's latest album, it's pretty nice," and then they end up liking it enough to buy the album. Even if that doesn't happen, though, nobody's lost anything. No, the value of the album hasn't really gone down - it's still going to sit at the same price of $15-$25.
How does common sense fit into all of that? Annoy people with money and power by not giving them what they think they should get and you'll get in trouble, I guess. But to me it seems like common sense to expect for something like that to happen given the technological situation.
It's late and I'm tired, so I hope that made sense.