1. Not promoting your own work. Your work might be fantastic, but if no one knows about it, it won't help your reputation, your salary, or your advancement opportunities. Make sure that your manager knows about your accomplishments, whether it's kudos from a hard-to-please client, waste you uncovered and fixed, or anything else that goes above and beyond your normal work.
2. Getting defensive. If you get defensive when you get less than glowing feedback on your work, you might be striking a death blow to your career. Many people simply give up on having meaningful interactions with defensive people, so your co-workers may avoid you, and your manager may stop telling you how you can improve. "That sounds great," you might respond—but it means that you'll destroy the relationships you need to advance in your career and denying yourself the information that you need to grow professionally.
3. Making rash decisions. Whether it's walking off the job because the boss said something you didn't like or taking a job offer without thinking it through carefully, impulsive decision-making has no place in your career. The decisions you make about work will have far-reaching ramifications on your wallet, your reputation, and your daily quality of life.
4. Not being assertive. You might think that not making waves is the best way to succeed professionally, but being unassertive is more likely to hurt you. If you believe a decision is wrong, or a project is headed for disaster, or that you deserve a raise, good managers will want you to speak up. There's a difference between being assertive and being obnoxiously pushy, of course, but voicing your opinions in a professional way is key to professional success.
5. Being too negative. If you're constantly complaining about new projects, your company's policies, and why it's taking IT so long to fix the network, you're probably creating an unpleasant environment for people around you. The same goes for negative humor—if you're regularly snarking about your boss or the new guy down the hall, chances are good that—even if people are laughing—you'll get a reputation for being bitter and having a bad attitude.
6. Lying. If you get caught in a lie—even if it's small or if it can't be proven—you'll destroy your credibility, and that's something you can never get back. You could be scrupulously honest for the next three years, but you'll still be remembered as the person who lied and can't be completely trusted.
7. Being chronically disorganized. People pay attention to whether you do what you say you're going to do, by when you say you're going to do it—whether it's as small as forwarding the document you promised in a meeting or as big as meeting a project deadline. If you do, they notice and you build a reputation as someone reliable and someone they can have confidence in. If you don't, they conclude that you can't be counted on to keep your word.
8. Not learning new technology. You might feel that you're perfectly comfortable with your existing ways of doing things, thank you very much, and therefore have no need to learn the latest technology … but if you resist new ways of doing things, you'll soon be left behind by colleagues who aren't so change-resistant. If you find yourself printing out emails to read them or heading to the library to look something up rather than Googling it, you're likely to be overlooked by employers in favor of your more technologically savvy competition.
Alison Green writes the popular Ask a Manager blog, where she dispenses advice on career, job search, and management issues. She's also the co-author of Managing to Change the World: The Nonprofit Manager's Guide to Getting Results, and former chief of staff of a successful nonprofit organization, where she oversaw day-to-day staff management, hiring, firing, and employee development.