1. Showing up way too early. It's good to plan to arrive early so you have a buffer against being late--but kill those last 20 minutes at a nearby coffee shop, not in the company's reception area. Many interviewers are annoyed when candidates show up more than five or ten minutes early, since they may feel obligated to interrupt what they're doing and go out to greet the person, and some (like me) feel vaguely guilty leaving someone sitting in their reception area that long. Aim to walk in five minutes early, but no more than that.
2. Underdressing. It doesn't matter if the office where you're interviewing is business casual; you still need to put on a suit and look professional. It signals that you take the job seriously. Sure, you might get hired if you wear a sweater and pants instead--I've hired people who wore that to the interview. But why wouldn't you want to play it safe and wear a suit? You can wear business casual when you're working there, after you impress them in the interview where you wear a suit. 3. Dressing like you're at a club. If anything you're wearing is see-through, has glitter, shows cleavage, or makes it hard to sit down without flashing the interviewer--that's a problem.
4. Flirting with the receptionist. By all means, be friendly to the receptionist; many interviewers ask the receptionist and other support staff for their impressions of candidates afterwards. But keep it professional: no giggling, talking about what you did last night, or asking for a phone number.
5. Asking questions that focus solely on salary and benefits. It signals that you're interested only in compensation and haven't put any thought into the details of the job, the company's culture, and so forth. Alison Green is chief of staff for a medium-size nonprofit where she oversees day-to-day management of the staff as well as hiring, firing, and staff development. She is working with the Management Center to coauthor a book on nonprofit management. Her writings have been published in the Washington Post, the New York Times, Maxim, and dozens of other newspapers. She blogs at Ask a Manager.