If you're like lots of people, you live in fear of making a big mistake at work--not just a run-of-the-mill error like everyone makes, but the kind that ends up making headlines and sending you away in shame.

An embarrassing and highly visible mixup may draw a lot of attention, but thanks to the fail-safe systems at most modern workplaces, they're actually quite rare.

The career mistakes you should worry about most are the ones that move more slowly--sometimes too slowly to notice. And they're deadly. Here are 12 of the worst:

1. Coasting. Maybe you've been at your company for a few years and you're feeling pretty comfortable in your job. That's not a bad thing in itself--unless you've stopped making an effort. You may think people don't notice, but they do. Make sure you do something every day to push yourself. Make an effort and put in the work, and be noticed for that instead.

2. Losing sight of the big picture. Sometimes the busiest and the most ambitious people get so lost in their projects, plans, and processes that they lose sight of the big picture. When you're constantly bogged down in detail, it's hard to step back and take in the wider view--but unless you do, you may be channeling all that work in a wrong direction.

3. Thinking the grass is greener. There are some who are always checking out other workplaces and even other industries, thinking that something new will be an improvement. Maybe it would be and maybe it wouldn't, but job hopping never looks good. If you think you may want to look elsewhere, be strategic and do your homework first.

4. Allowing your skills to become out of date. Especially with technology advancing so rapidly, it's extremely important to stay on top of your game and not allow your skills to become outdated. As you grow older, you don't want to be labeled as someone who can't keep up. Take classes, read, keep your professional network strong--whatever it takes to stay current.

5. Not believing in yourself. If you don't believe in yourself, how do you expect your boss, colleagues, and clients or customers to believe in you? Do everything you can to build up your confidence. Start by shutting down any negative self-talk. Remind yourself instead of your strengths, gifts, and abilities.

6. Burning bridges. When things go seriously wrong, it's always tempting to say, "I'm out of here" and walk away. Leaving may feel better in the moment, but it ends up costing you later. Learn to stay and work things out. Far better to have the reputation as someone who's tenacious than to be seen as a quitter.

7. Not trusting your gut. Einstein once said that every major discovery he made happened when he left his rational mind behind. If you don't trust your instincts, you're cutting yourself off from a powerful source of creativity and innovation.

8. Wasting your energy on the wrong things. It's easy to get caught up in being busy but not being productive, working hard but not bringing value. Be strategic. Don't chase shiny objects--keep your focus on the projects that will drive the most value and move your organization forward.

9. Understating your worth. Don't be afraid to show that you know your worth. Ask for raises and promotions when you deserve them, and overcome any discomfort with marketing yourself and your contribution. Be genuine and generous toward others so you don't come off as arrogant.

10. Always seeing the glass as half-empty. The strongest organizational cultures don't have much room for pessimists who always notice when things aren't working. That doesn't mean you have to overlook problems--just make sure you stay focused on solutions and maintain a positive attitude.

11. Job hunting on company time. Get serious. Your online history at work is never private--never mind the danger of email mishaps or someone walking within view of your screen (or within earshot of your phone conversation)--and there are few things you can do that will damage your reputation more.

12. Thinking your job will last forever. Any job can come to an end at any time, for all kinds of reasons unrelated to your performance. Stay prepared for every contingency by giving your best where you are and staying prepared to move to a new opportunity if the worst happens.

Keep your career tuned up and moving forward, and live your work life (and your personal life, too, for that matter) free of regret.