What are the top 7 deadly sins of job interviews?

I agree with William, and add the following.

Inappropriate dress.
Poor personal hygiene.
Lack of eye contact.
Not being able to communicate or demonstrate why you are the right or best person for the position.

Thanks for the interesting question and useful answers. But the presumed perspective so far is from the interviewer judging the interviewee. The question could just as appropriately provoke responses from the Candidates about Interviewers &/or prospective Employers. I have been involved in all three roles and think that each participant needs to be aware of the perspective of the other: Employer, HR Advisor & Candidate. From the candidate's perspective I would add:
a) not giving candidates enough information about the interview process and the panel composition;
b) not being clear about the position and misleading candidates about the job;
c) having unrealistic expectations of the candiate pool;
d) not keeping to a reasonable timetable (not allowing enough time for the candidate and post-interview documentation before the next candidate);
e) running late without notifying candidates;
f) not treating the candidate with respect;
g) sticking too labouriously to the set questions, and not exploring initiative or innovative approaches.
I hope that promotes more interesting discussion?
Best wishes - Dr Colin A Sharp

Here's a few that come to mind for me:

1. Answering your cell phone during an interview.
2. Not researching the company beforehand and having thoughtful questions prepared.
3. Not doing your homework on yourself!

Here are 7 sins that have irritated me as an interviewer in management consulting:

1) Showing up too late
2) Avoiding eye contact with the interviewer
3) Lack of knowledge about the prospective employer
4) Arguing with the interviewer about facts on the interviewer´s CV
5) Unability to highlight a certain competence or experience that would give a spike in the candidate´sprofile
6) Unability to bridge own experience with competences needed to get the job
7) Finishing the interview without asking a single question

My top 7 in no particular order:

1.) Dressed in a non-professional manner (i.e. Jeans, t-shirts, ripped up clothes)
2.) Being late to the interview unless it is by something out of their control (i.e. weather related, car accident)
3.) Talking in slang (I had a person come in once and say 'Wassup, yo?' to me)
4.) Not being able to show why they are a good fit for the company and why the company is a good fit for them (it works both ways)
5.) Lying during the interview or on their resume, especially about their skills and abilities
6.) Leaving their cell phone on or answering it (it happens more often than not)
7.) Discussing what they did not like about their previous manager, position or job (because let's face it, every position has something you don't like about it)

Other things include smelling strongly of any scent (whether perfume or bad hygiene), poor body language and not being able to tell me why they were interested in this particular position.

My answers are from the HR and/or Company perspective:
7. Not considering your appearance - be sure to look your best including clean nails and shoes
6. Being too early or late - be punctual without arriving more than 15 minutes before the scheduled time as this is almost as bad as being late
5. Having a poor handshake - look the person in the eye and shake confidently (no wimpy handshakes - and don't squeeze)
4. Talking too much / too little - I've seen as many candidates talk their way OUT of a position as I've seen be too quiet and not ask questions or sell themselves; stick to answering the questions without adding inappropriate stories
3. Ask about pay during initial interview - this question should be reserved for the Recruiter/HR person you are dealing with; further, you should know the pay range BEFORE you interview
2. Little or no research on the company - you should know what their core business is and, at certain levels, you should know about their financials
1. Falsifying the resume/application - many companies today run back-ground checks so we'll know if you don't actually have a degree or only worked at a company for 3 months (vs 3 years); this would be the deadliest of the sins

Well there are the deal breakers in the first 3 seconds of meeting someone. Then there are the actual issues during the interview.

1) Turning up late with a poor excuse.
2) Not coming dressed to fit the role for which you are interviewing.
3) Poor personal hygiene.
4) Limp handshakes.
5) Over assertive handshake (bad - step in push the interviews hand down to show dominance)
6) Poor body language, crossed arms, sitting far back in the chair.
7) Lack of eye contact, during the interview.
8) Leaving your mobile phone on. Checking your mobile phone if I leave the room.
9) Not understanding the role for which you applied.
10) Inaccurate information on your CV.
11) Lack of passion or personality.