Earlier this year, career coach Kate Wendleton found herself advising a man whose firing was entirely avoidable: He mistakenly e-mailed an offensive joke to a client by pressing "reply to all" instead of just "reply." The offense was so grave that the man lost his job.

With e-mail now such a common office tool, you'd think that workers would have learned to be more careful -- especially as most people have either heard horror stories about e-mail snafus, or have committed an electronic peccadillo themselves. But careless use of office e-mail still is getting a surprising number of workers in hot water.

E-mails have contributed to New York Attorney General Eliot Spitzer's probe of the insurance industry, for instance. And e-mails regularly figure in other lawsuits. "People tend to think of e-mail as being secured communication between the recipient and the person who's sending the e-mail -- that's not the case," says Mary Casey, managing attorney of the Harbor Law Group in Shrewsbury, Mass., which specializes in business law and intellectual property. "It's not a private communication."

Even less-egregious miscues can cause damage and embarrassment. Consider a beauty-products company whose customer-service department accidentally forwarded employee comments to a customer. A woman had e-mailed the company complaining that her dog had eaten the company's shampoo and gotten so sick he required medical attention. She asked the company not only to pay her vet bills, but for additional payment to compensate for her emotional distress.

The customer-service staffer forwarded the e-mail to a legal staffer, who pooh-poohed the request and wrote "this woman's an idiot," recalls a consultant to the company who saw the e-mail. The customer-service staffer accidentally included the legal staffer's derogatory comments when she replied to the customer. The customer was so furious she contacted the company's chief executive officer. The firm wound up paying the woman several thousand dollars.

"E-mail is really dangerous sometimes," the consultant says. The company now responds to customers with phone calls or regular letters instead of by e-mail.

Administrative assistant Steffanie Roulet was working for an executive recruiter a few years ago when she committed an e-mail error that still makes her cringe. She meant to send an e-mail to about 15 executives asking if they were interested in a particular job. Instead of sending it to each candidate individually, she sent the e-mail to everyone at the same time. As a result, each candidate saw who else was being considered and who in their industry was job-hunting. "I hate my life right now," she recalls thinking.

She immediately told her boss what she had done, and thinks her quick confession helped save her job. "If he had found out without my telling him, I am confident that he would have fired me," she says. Instead, she got "a full explanation" of the disasters she could have caused and endless teasing from co-workers. "There were a lot of potentially major nasty fallouts," she says. "I just was really lucky that I didn't cause any board to go and fire people."

Susan Joyce, who runs job-hunt.org, a resource for online job-hunting, once accidentally sent an embarrassing e-mail to a group of co-workers, instead of one colleague. Her boss's secretary had sent the group an e-mail about shared lodging arrangements on a business trip. Ms. Joyce, thinking she was sending an e-mail just to the secretary, jokingly wrote that she didn't mind sharing a room, but her roommate might because "my husband says I snore a lot," Ms. Joyce recalls.

Now she recalls that experience when giving advice to job-seekers. She follows an e-mail "golden rule": "Don't hit the 'send' button unless you would be comfortable having your mother, grandmother, or a competitor reading what you just wrote on the front page" of a newspaper, she says.

Ms. Joyce takes other precautions as well. "When I'm putting something into an e-mail to a client, I pretend I'm putting it on letterhead," she says. After all, if an e-mail comes from a company e-mail address, "effectively it's going out on company letterhead, it just doesn't look like it when you're typing it."

Ms. Wendleton, president of the Five O'Clock Club, a national career counseling organization, tells job-seekers to use the same kind of formality in e-mails to prospective employers as they would in a written letter. She tells people to compose such e-mails in a separate document, print it to check over, and then put the text into an e-mail. "Exercise some caution," she says.

At her firm, she has a policy that employees always should have another person look over external e-mails, unless it's a routine form letter. It doesn't have to be a supervisor, just another set of eyes, Ms. Wendleton says. When writing e-mails herself, Ms. Wendleton takes care not to continue an e-mail conversation that could look bad later if forwarded to someone else. She'll send an e-mail saying: "I hesitate to write this in an e-mail; why don't we talk over the phone," she says. "Things can be passed around."