Your race or questions relating to your color or national origin. Your height or weight - where it is not relevant to the job requirements. Whether you have ever been arrested. Political or social affiliations. The way you respond to an inappropriate question could play a vital role in whether or not you get a job offer.
Most interviewers ask illegal questions out of ignorance or lack of understanding of employment law.
Discrimination, although illegal, is very difficult to prove in court. The best way to deal with an illegal question is to identify the real concern of the employer and answer that concern . For example, if a woman is asked if she has children, the real concern would probably be that of reliability on the job. She should assure the interviewer that child care is not a problem and will not interfere with her ability to do her work. Likewise, the question pertaining to marital status shows concern for whether you will be staying in the area very long.