I like the "well, I don't know, but if you give me a few minutes with an internet connection" response to questions like these. =)
Seriously, though -- this is something that, as one of the primary people responsible for hiring at my company, I struggle with a bit. We tend to interview much younger talent (e.g. just out of college, or sometimes even still in high school, especially for co-ops/internships), so coming across as an arrogant know-it-all is even easier given the experience gap.
To try to address this, we've made a ridiculously long pool of questions and then we hand-pick from it as we establish more of an understanding of the candidate.
We have a couple of baseline questions almost everyone hears; some are even sent before a verbal interview as general screening questions. (These are the kind that are fairly simple and implementable in almost any language -- we're more interested in the specific choices people make and the structure of the response than anything else, but it does serve to eliminate some candidates occasionally.)
Especially with younger / less experienced candidates, I try my best to make the interview comfortable, and encourage the interviewee to ask questions. This may not be as "hardcore" as other interviewing methods, but by in large I find that you can still ascertain if there are serious problems. (The issue here is trying to avoid coming across as condescending.) And it gives the candidate a chance to shine.
There are always ways to knock people off their guard in interviews. (See e.g. Jeff Atwood on puzzle questions if you're feeling mean, or Joel Spolsky on impossible questions.) And yeah, you've probably got to try to throw people for a loop at least once in an interview to see how they react.
But generally it's a lot harder to see a candidate's abilities under near-optimal conditions, and that's something that I'm interested in. Subsequently, you have to answer "how hard it is to get that person to work at that level"; it might not be possible.
This place may be somewhat different in that we don't deal with a tremendously large volume of applicants and we have a sort of "social contract" to give back to our applicants to a certain degree (to help local colleges, community, etc). So this might not be something everyone can do.
But I stand by my assertion that only throwing interviewees off guard or making them miserable is not going to give you a remotely complete picture -- and that's the interviewer's loss.