If you say you have a second job, the employer may think you are not fully committed to this job or are having financial difficulties or other perceptions that you are not a good candidate. People often get into real estate and may even list it on their resume that they have a license. It has nothing to do with the job you're interviewing for. It only adds to the employer's perception that you are not committed and you will not be there when they need you. Plus, they'll assume that you'll receive a lot of calls from potential home buyers at work.
Some people fib by saying they are going to school and that limits their ability to work overtime.
Truth is, they have another job. And, if the employer checks so see if you are enrolled at the school, and you're not, that could be grounds for termination. If you ARE going to school, that is usually viewed positively and companies are willing to work with your schedule. So, it's easy to give the employer ammunition for disqualifying you for a job. Talking about anything that doesn't relate to the job could give the interviewer a negative impression about your ability to perform the job.