You messed up a job search, making a faux pas during an interview or handling a turndown badly. But you probably don't realize that your mistake, exacerbated by the tight job market, could harm your long-term prospects.

At a networking event last August in Bellevue, Wash., a recruiter pointed to a software developer across the room. He's qualified, but "very bad in his presentation skills," he told career coach Paul Anderson and a human-resources official for a big technology concern. "What's that guy's name?" Mr. Anderson remembers the HR official asking, and then scribbling the name in her notebook. "I want to add him to our blacklist."

The developer then walked over to the trio to inquire whether the recruiter had found relevant openings for him. The recruiter replied he was still looking. But once the job seeker left, the recruiter "told us he would never submit him to any clients," Mr. Anderson recalls.

The developer unwittingly landed on two "do not hire" lists that day—a far from unusual occurrence. While U.S. search firms and hiring managers rarely admit they have such tallies, a growing number keep unofficial blacklists of undesirable applicants. These individuals often remain untouchable for years.Recruiters and employers mainly want to exclude liars, losers and misfits. Yet you also can get banned for minor infractions, such as simply taking a counteroffer. And it's hard to discover or remove a bad mark beside your name.

Mr. Anderson, head of ProLango Consulting Inc. in Redmond, Wash., canvassed two dozen recruiters and employers last year about blacklists. "Every single one of them kept track of candidates they rejected for employment—in their computers or their heads," he reports.

While a program manager for Microsoft Corp. until late 2006, Mr. Anderson says he blacklisted certain prospects after his preliminary screening found they had exaggerated their qualifications on their résumés. Mr. Anderson says he and his team had hiring folders on Microsoft's intranet, and these prospects' names "were put on the 'do not hire' list'' in those folders.

Microsoft says it doesn't maintain such lists. "The only time an applicant would be flagged is if they have failed a background check," a spokeswoman says. "Any company would want to make note of such failures in their system." Those individuals can't re-apply for a finite period, which Microsoft handles case by case, she explains.

Many snub-worthy mistakes occur as a result of job hunters' anxiety, which is being amplified by today's high unemployment. An out-of-work software engineer gave Walden Recruiting of Concord, Mass., permission to submit his résumé to a Boston Internet company in late 2008. But he secretly let a rival search firm do so first, according to Marsh Sutherland, Walden's president. That meant Walden wouldn't collect a $24,000 fee, notes Mr. Sutherland. "I was very angry," and yelled at the engineer for lying, he says.

The engineer told him he was trying "to increase his chances of getting interviewed." Walden and the Internet business blacklisted him, says Mr. Sutherland. The engineer nevertheless sought more referrals from Walden—without success.

The engineer agrees he felt scared about seeking work during the holiday season and "didn't control the situation" well by dealing with two search firms simultaneously. But he denies giving Mr. Sutherland the right to submit his résumé first. The recruiter's refusal to help him further "was tough," adds the engineer, who found his latest full-time job last March.

So how do you find out if your job search is being derailed by an offense you unknowingly committed? Your career coach may be able to learn if an employer labeled you unsuitable.

Last July, a ProLango staffer contacted a major defense contractor on behalf of a promising client. But an in-house recruiter refused to consider the programmer because the official remembered asking him during a 2007 interview there about undergoing a drug test. "Sure! As long as you give me six days' notice!" he joked. The rejected programmer now says he hopes the defense contractor "puts some kind of time limit on silly comments made, and not hold it against me for the rest of my life."

Getting back into the good graces of a recruiter or hiring manager might require going the extra mile. A senior partner at a global executive-search firm placed a red flag in a prospect's computerized file at the firm when he broke his promise to accept the international presidency of a major retailer in 2008. This was after extensive efforts by the retailer's CEO to work out an attractive pay deal for the executive, who even signed an agreement to join. The hospitality-industry executive instead advanced to the No. 2 spot at his current employer.

"I didn't speak to him for a year," recalls the infuriated recruiter, who marked the executive's file with "conversation required"—as in "talk to me before recommending this person to another client." The search firm didn't arrange interviews for the executive elsewhere.

In 2009, however, the executive offered the recruiter highly detailed information about the record of an acquaintance who was a candidate for a different job and conducted extra reference checks. That man got hired and flourished.

The upshot? The recruiter dropped the "conversation required" tag last month.

Copyright 2009 Dow Jones & Company, Inc.