When can asking a question during lecture be offensive?
Asking questions is a good thing, but you need to make sure that the questions you ask are appropriate. The best thing to do is judge for yourself whether or not a majority of the audience might be interested in your question.

An example might be that the lecturer has made a mistake, or has forgotten to define something in a mathematical derivation. In such cases, perhaps you could whisper to the person next to you for a quick second opinion.

If a concept has been neglected or poorly explained, then you should ask for expansion or clarification if you think that most of the class will be in the same boat.

These aren't always easy judgements to make. If you're uneasy, save the question for after class. You can also simply ask the lecturer if he or she minds you asking questions, or your classmates if they thought your question was a good one. Good questions enhance the learning experience for everyone.

The bottom line is that most good professors like to have some interaction with the class. Many students audience don't like to say anything, so it's often up to just a few students to ask any relevant questions. But more than one question every ten minutes or so can start to get tiresome for the lecturer, so unless it's a tutorial class you shouldn't turn it into a Q&amp;A session.