Getting distracted and missing a question looks bad on your part. If you zone out, your potential employer will wonder how you will be able to stay focused during a day on the job, if you can't even focus during one interview.
If you feel your attention slipping away, make the effort to stay engaged. Maintain eye contact, lean forward slightly when talking to your interviewer, and make an active effort to listen effectively.
While you may have no problem paying attention in a one-on-one interview in a private office, it's harder to stay in tune with the interviewer when you're meeting in a public place. Read more about tips for interviewing in a public place.
In stressful situations, as interviews often are, people tend to talk too much. That's the next interview mistake to avoid.