“Tell me about when you offended somebody.”
This kind of question is not unusual, at least in my experience. It depends on the job in question, but often, tough or delicate questions are asked to see:
- How does the candidate react to delicate questions? Does she/he stay calm, even-tempered, cool-headed? Is she/he up to these questions?
- Does the candidate disclose more informationsinformation about her/his character or personality than she/he wanted to? Does she/he do a "soul striptease"?
Interviewers try to elicitateelicit as much information about your character as they can. Delicate questions are a good way to do this.
In your example, a candidate could talk herself/himself into trouble, if from the answer the interviewer for example gets the impression of an aggressive person who likes to tangle with others.
So how should you answer that question?
You cannot prepare for all possible questions. But you can prepare for delicate questions in general. Stay calm, don't reveal such situations (in this case, situations where you offended someone), even if this happened to you (and wasn't a harmless situation). You don't want to risk to cast a poor light on yourself. Show that you behave professionally all the time.
You could for example say "This never happened to me. I try hard to not offend others. If there is a conflict, I try to clarify it objectively and unemotional."
Update: Of course, you can admit harmless situations like misunderstandings which have been clarified easily.
p.s. English is not my native language. I hope I have found words with the correct meaning.