1. Stealing
Whether you think that the company “owes” you things like Post-its, pens and other office supplies, or you simply don’t think it’s a big deal, you should be sure about that in case your boss or co-worker catches you. Stealing can not only get you fired, it’s illegal.
2. Blaming someone else for your mistake
It’s dishonest, childish and tacky. Very tacky. Sure, having to admit you made a mistake is embarrassing, but not nearly as embarrassing as it would be to have to admit you lied about it or tried to cover it up.
3. Spreading gossip
Who knows why people get a certain high from hearing juicy gossip. Many of us are guilty of passing along at least a few rumors at times, but the more you can avoid doing so, the better off you will be. Try to avoid it. In addition, if you hear it, don’t repeat it.
4. Calling in sick when you are perfectly healthy …
If you wake up and just cannot face the idea of going in to work, there’s no reason (unless this happens every day) you shouldn’t give yourself a vacation, personal or mental health day. Nevertheless, calling in sick is not only irresponsible and insensitive to the co-workers who have to cover for you, it could also be grounds for firing if you’re caught.
5. … Or showing up when you aren’t
The admiration your co-workers show you for coming in to work despite having the flu will be short-lived: It won’t take long for those around you to tire of hearing hacking, sniffling and coughing coming from your desk, nor will they appreciate being exposed to your germs.
6. Abusing office technology
This includes (but is not limited to): spending a significant amount of time on personal phone calls, e-mails or instant messages; using office time to type and print out the pieces of your screenplay; going online to catch up on last night’s episode of “My Life on the D List”; and using the office printer to make the invites to your holiday party.
7. Involving co-workers in your personal problems
If problems at home are distracting you from doing your job, that’s one thing, which you should take up with the boss or human resources. But involving your co-workers in your personal problems is distracting and unprofessional.
8. Getting too comfortable
Dressing professionally is just as important as behaving professionally in the workplace. No matter how long you’ve been with the company, how “cool” your boss is, how much sleep you got the night before, or how cold or hot it is outside, you should always maintain a clean, workplace-appropriate appearance. Even if you work in a relatively casual office, you should still make the effort to wear clean clothes and have well-groomed hair. A good rule of thumb is to never dress better than your boss, nor worse.
9. Hitting on your boss
This one has “Lifetime movie” written all over it. For every successful office romance story, there’s a disaster story to match it. When co-workers hook up, it always makes for a tricky situation; when one worker is subordinate to the other, the stakes get even higher. Unless you are well aware of your employer’s policy regarding office romances and are prepared for the possibility of rejection, avoid this one altogether.
10. Hitting on your employee
Consider all of the above, and add to that a possible sexual harassment suit.

By Mary Lorenz, CareerBuilder.com writer