The most common "worst" thing I see on a fairly regular basis is when a candidate goes on and on about how bad his/her current employer is and how horrible it is at XYZ Corporation and how he/she just cannot wait to get out of there etc.
First impressions are everything and you want to be sure to represent yourself well!
It is not about being a "cookie-cutter" employee, but it is about showing a level of professionalism and respect for both yourself and the individuals who have taken the time to speak with you about their company and the opportunities therein.
It is important to have a good rapport with the interviewer but make sure you treat the interview like an interview. You will not want to come across as somebody that is having a dialogue with a friend.
[O]ne of the worst experiences I have had was when a woman I was interviewing decided to go into graphic details about a personal health situation she was having. It was incredibly uncomfortable and when I acknowledged that I was sorry she had that experience and tried to move the interview forward, she launched into more and more graphic details about her situation.
Being prepared will help you from pausing too frequently and saying "Um" too much.
[E]ven though an interview is your time to brag about yourself a bit, you also should be humble and clearly able to answer the questions of the interviewer and articulate specifics.
It bothers me when a candidate has their cell phone going off during an interview, and I remember even once, was asked if I minded if they took the call.