Unfortunately, this is a huge one. Not taking the two seconds to look at your phone and make sure it's silent before something as important as an interview shows a basic lack of care and understanding that can't be won back.
2. Looking less than your best
Even though the dress code for many companies hiring Millennials is casual, that's no excuse for looking like a slob when you show up for your interview. I've always been told that you should dress better than the person who is interviewing you, and I think it's a good rule to follow when you want to impress.
3. Blasting your previous employer or company
Although the environment around the Millennial work force is much more casual than with previous generations of workers--in fact, some say that the openness is something unique to this generation--it's still necessary to have a professional attitude. Talking bad about your current or previous employer makes you look bad, and the interviewer is going to wonder how long it will be before you start talking bad about them.
4. Showing up late
Showing up late for a job interview is inexcusable. It conveys a basic lack of planning and a disorganized state. You never know when your car will decide not to start, or when a clogged freeway will slow you down, or when the subway will be stuck waiting for traffic to clear. Plan to arrive early--15 minutes wouldn't put you too far out of your way--to ensure that you're on time.
5. Displaying a bad attitude
Some people are naturally explosive, emotional, and love to share. In this way, their bad attitudes are practically uncontrollable and difficult to contain during the course of a long job interview. But contain them you must. Think good thoughts, and push down any negative emotions you might have until after the interview.