1. Not applying for the job properly in the first place. Like emailing the wrong person, or not using the online system they set up for applications. Just read the job description carefully, and do exactly what they say.
2. Not being fun enough in your interview. It depends on the job. But in general, we like working with people we enjoy being around. So if you come off as rigid and boring,your chances go down.
3. Being TOO FUN. Some people overdo it and act like they're talking to a friend instead of a potential boss. So you want to be likeable, but also professional. Not like you're hanging out at a bar with friends.
4. Still using an embarrassing email address. But even just using the wrong email host can make you look bad. In general, sites like AOL, Excite, and Yahoo are seen as less professional. Gmail is the new standard.