BANG! Without warning, the rifle discharged, tearing a hole through the floorboard of the car of a US Army colonel. The rifle belonged to a young lieutenant who had been invited to go hunting with the colonel.

Though no one was hurt, the incident left everyone in the car shaken. Worse, the lieutenant hindered his own promotion, according to executive coach Bruce Sillers, who was a member of that same battalion at the time of the incident.

You may never have committed as grave a faux pas as this lieutenant, and if so, be thankful. Nonetheless, we're all capable of making mistakes that can send us straight to the career doghouse. Here are five big no-nos to watch out for.

1. Sending inappropriate email

Most of us are bright enough to realise that chain letters or off-colour jokes have no place in business communications. Where most office workers get into trouble is with the over-hasty email reply.

Ever read an email too quickly and fired off an angry reply, only to discover later that you had misinterpreted the first sender's message? You end up not only wasting everyone's time, but poisoning your work relationships -- perhaps permanently.

Before you reply to an email that has elevated your blood pressure, apply one of these useful tests: Ask yourself, "Would I feel comfortable explaining my response on a witness stand?" or "Would I want my response to be published on the front page of The New York Times?"

If the answer is no, take time to cool off. Store the message in a drafts folder and review it later. Are you sure this is what you want to say, especially if you're directly insulting the recipient? Can your words be interpreted more negatively than you intended? And finally, would you want this message to find its way to your boss -- or to the HR director?

By the way, don't rely on any "unsend" feature, either. That feature will fail when you need it most. And be very careful of hitting Reply All, or your supposedly personal conversation could be the talk of the office.

2. Putting down co-workers

Having done a significant amount of work for a particular client, I decided one day to try to expand my presence there. I called an executive in another part of that organisation, introduced myself and said that "Carl" (a fictitious name for the IT executive with whom I had been working) was pleased with my work.

That executive responded, "Why should I care what Carl thinks?"

Not smart -- especially when said to someone outside the organisation. If Carl had heard about this remark -- and these things do get around -- it could have created a Grand Canyon-size rift between him and his indiscreet co-worker. More critically, remarks like this damage the credibility of the organisation.

Here's another example: Suppose you're the person the help desk elevates problems to when they are unable to resolve them. You find out, while talking to a customer, that the staffer she talked to gave her some really poor information. At this point, you may think the staffer is an idiot, but it's not a good idea to say so.

For one thing, if word gets to your boss that you're bad-mouthing your co-workers to the customers, you could be in big trouble. CIO Denny Brown of electric utility provider Arizona Public Service makes no bones about it: Such behaviour constitutes insubordination, and therefore is "grounds for termination," he says.

It's a much better idea to maintain a united company front when dealing with the customer. Resolve the issue with your IT colleague privately.

3. Contradicting the boss in public

Suppose that your boss, while giving a presentation, makes a factual error. Should you jump in and correct the error immediately, secure in the knowledge that your boss will thank you for underlining the mistake in front of an entire room of people?

Um ... no.

Correcting your boss in public will hardly endear you to him. More likely, he will be upset at being made to look foolish, and may even wonder why you didn't catch the error yourself prior to the presentation.

When may one safely contradict the boss in public? I can think of only two instances:

First, if the building is on fire and your boss is pointing people to the wrong exit, you probably can speak up with few repercussions.

Second, if the boss makes a mistake about making a mistake, you can speak up -- the louder, the better. So, if your boss identifies the correct vendor for your off-site backup, then mistakenly says, "Sorry, that was wrong," you absolutely may say, "No boss, you were right to begin with." You don't get these chances very often, so take advantage of them.

Otherwise, exercise extreme discretion when your boss misspeaks in public. If the matter is truly important (for example, the CIO gives the wrong date for your SAP go-live), approach him during a break and quietly mention the mistake. A smart and gracious CIO, upon resumption of the session, will identify the error, apologise and credit you with the correction.

If a break isn't forthcoming soon, try to catch your boss' eye and talk privately. But you really don't want to shout out the correction in front of the whole group.

4. Committing social blunders at a company event

Staff misbehaviour at office parties has been a cliche since the 1950s, but that doesn't mean people still don't make fools of themselves. Don Michalak, co-author of Making the Training Process Work and a consultant for companies such as Ford, KPMG and Marsh & McLennan Co., stresses that such functions are not purely social events. "Don't do anything you wouldn't do at the office or at a client's office," he says.

Yes, the party will have food. Go ahead and eat some, but don't draw attention to yourself by parking at the shrimp cocktail table. (Right or wrong, people will judge you if you pig out.) Consider eating something before you get to the party to avoid looking famished when you arrive. Be careful if the party offers alcohol; you know what can happen when a person drinks too much.

If you bring a guest, ask that person beforehand to be careful about his words. You don't want your guest to say to your boss, for example, "Oh, you're not as bald as they said you were!"

By the way, no matter how well you get along with your co-workers, the party is no time to complain about all the overtime you had to put in on the SAP rollout. If you do talk about the hours or the project, try to keep things positive, as in, "It was tough, but we did it."

5. Burning bridges when you resign

Many of us fantasise about telling off the boss when we quit a job -- but before you let loose, think twice. Remember the '90s Internet bubble? Many IT people left traditional companies with visions of pulling in millions from Internet start-ups, only to be rudely surprised when their new companies went under. Those who left on good terms with their former employers had a better chance of being rehired.

Christian Bass is a firm believer in maintaining good relationships with previous employers. Until 2006, Bass served as director of academic technologies at George Washington University. After leaving GWU, he spent two years as an employee of a consulting company; he then formed his own company, Successant LLC, in 2008. He recently negotiated a consulting contract with -- you guessed it -- his old boss at GWU.

When asked how he handled his GWU resignation, Bass said he emphasised that he was leaving for positive rather than negative reasons. "If something was bothering me at work," he said, "I resolved it, rather than letting it be the factor that led me to leave." He also stressed the importance of leaving with a good reputation and a record of solid accomplishments.

So, when you leave, keep things as gracious as you can. When you make the Big Announcement, stress the advantages of the new job, not the shortcomings of the current one. Conversely, come up with reasons to be grateful to have worked at the latter, but be sincere and don't make things up.

If you learned something from your boss or co-workers, let them know. Even if you had difficulties with someone, you still could say, "Thanks for teaching me how to benchmark an Active Directory environment." Leaving on good terms can only help you if you encounter these folks later.