Beyond situations in which telling the truth can directly influence someone to be a danger to hirself(him/herself) or others, and other situations regarding the law (because those aren't interesting or thoughtful answers)
The potential answers to that question are different between cultures. For example, in many foreign nations, it's culturally impermissible to tell someone they have cancer (but you can tell their families... who will most likely hide it from them), not because of any kind of legal issue, but because the culture as a whole believes that people are more likely to die if they accept that they're going to die and act like they're going to die - keep them in good spirits, and they'll more likely improve.
"Does this make me look fat?"
You don't say that to someone. That causes more harm than good.
Think of it in a practical way - what would cause more harm than good? Not just to yourself (in that you're less likely to get slapped in the face after calling your girlfriend fat), but to everyone involved (she won't have a good time tonight, she'll hate herself and you, and "does this make me look fat" is a question to which the most culturally accepted answer is "no, of course not, sweetie." Not saying it's always best to lie, because some relationships are such where "well, maybe you'd look better in something else... this kind of accentuates your cellulite" would be an appreciated piece of advice and not an insult)
If asked this at an interview, PLEASE don't just leave it at "you don't tell someone their lab results are the worst you've ever seen, you put it more lightly." Make it more interesting. Don't be a bland and uninteresting cookie-cutter interviewee.