Refusing to sit down may annoy your interviewer.
Taking calls from other headhunters during your interview could be construed as bad manners.
Turning up drunk, late, on the wrong day does not always give a good impression.
Don't try coming on to your interviewer - at least not during the interview.
Eating your breakfast-to-go or your packed lunch during the interview might give you spinach-in-the-teeth.
Don't get aggressive - an interview is not a battle - and try not to swear.
Bringing a portfolio of work that is not yours is dodgy in anyone's eyes.
"I'm really pleased with the quality of candidates that you've been sending over. Absolutely spot on. It's a great help having someone so efficient working with us, so thank you."
- Alastair Peck, Mercer, 2011