By Caroline M.L. Potter

Twitter is a must for many careers now. Some employers even insist that you have a certain number of followers to be eligible for media jobs. But there is a dark side to this medium.

Its immediacy and casual nature (how formal can one be in 140 characters?) have caused a number of professional slip-ups. To protect your career and online reputation, be sure not to commit any of these Twitter don'ts:

Don't Tweet Your Way Out of a Job Before the Start

Connor Riley, a graduate student at the University of California-Berkeley, took to Twitter after receiving a job offer from Cisco, tweeting, "Cisco just offered me a job! Now I have to weigh the utility of a fatty paycheck against the daily commute to San Jose and hating the work." She received a reply from a Cisco employee, "Who is the hiring manager. I'm sure they would love to know that you will hate the work. We here at Cisco are versed in the Web."

Riley's blog was bombarded with negative comments, and she never wound up working at Cisco. Regarding the backlash, she later told MSNBC, "I was just looking at summer internship positions. It's perfectly reasonable for some people to be extremely offended that someone would be so flippant about a job in such a bad job market."

Don't Forget That Even a Personal Tweet Is a Client's Business

While working for public relations firm Ketchum, James Andrews traveled to Memphis to meet with FedEx, a client of the firm. Before his presentation, he tweeted, "True confession but I am in one of those towns where I scratch my head and say 'I would die if I had to live here.'"

A FedEx employee found the tweet and shared it with upper management, who are very loyal to the city of Memphis, and managers at Ketchum. FedEx promptly issued a lengthy dressing down to Andrews, who no longer works at Ketchum.

Don't Use Twitter to Negotiate with Your Employer

On the eve of the first auditions for the ninth season of "American Idol," judge Paula Abdul tweeted, "With sadness in my heart, I've decided not to return to #IDOL. I'll miss nurturing all the new talent, but most of all being a part of a show that I helped from day1 become an international phenomenon." Some speculated that this was a contract negotiation tactic; but if it was, it didn't work.

The next morning, Fox and producers FremantleMedia North America and 19 Entertainment issued a statement saying that Paula Abdul was "an important part of the 'American Idol' family over the last eight seasons and we are saddened that she has decided not to return to the show." Ellen DeGeneres stepped in as a judge instead.

Don't Use a Tweet to Settle a Beef

Alice Hoffman, author of Practical Magic and Here on Earth and not someone you might expect to have a misstep involving the written word, did just that when critic Roberta Silman gave one of Hoffman's books a tepid review in the Boston Globe.

The novelist took to Twitter to trash Silman. Not once. Not twice. Not thrice. Rather, Hoffman sent out 27 tweets in which she called Silman, among other things, a moron and revealed the critic's phone number and email address, urging her followers to "tell Roberta Silman off." Shortly after this feud, Hoffman deleted her Twitter account.

Don't Blow Your Cover with a Tweet

Lawmakers are often in the news for their own missteps. When former Rep. Pete Hoekstra (R-Michigan) visited Iraq in February 2009, he promptly tweeted his whereabouts: "Moved into green zone by helicopter Iraqi flag now over palace. Headed to new US embassy Appears calmer less chaotic than previous here." MSNBC pointed out that he was fortunate his followers didn't include "guys who may wish America ill."

That same month, Jeffrey Frederick, then-chairman of Virginia's Republican party, sent out this tweet: "Big news coming out of Senate: Apparently one dem is either switching or leaving the dem caucus. Negotiations for power sharing underway." Democrats were following his stream and, according to MSNBC, negotiations came to a standstill.