Don't be too confident
Confidence is good but being overconfident can be detrimental. At times, it can push you to talk out of order or act out of control. Also, don't assume that you have got the job until you really get it in written with due approvals.
Don't arrive too early
Being punctual is a desired quality that employers look for in job seekers. However, arriving too early can leave a negative impression on the interviewer.
Don't smell bad
No matter how great you are in the job interview, if you smell bad, chances are you will lose the job. Don't smoke or eat odorous food before going for an interview.
Don't be rude
Obviously you will be good to the interviewer but you must be polite with other people also you meet during your interview. If you are rude to anybody and it gets reported you may just miss out on the job opportunity.
Don't check social media
Don't risk losing your focus on errant things on social media. Focus on your interview and keep that smartphone away till the time being.