They misrepresent you to the prospective employer. They misrepresent the prospective employer and the position to you. Typically they do so by selective omission; they only list the best things about you or the job to the other party, to give the impression that you must match in all other areas, too.
If one is following the low road (and many
pimps headhunters do), there is every incentive to do so, because having once engaged in representing you to an employer (no matter how poor the match, and misrepresenting encourages both you and the employer to show initial interest even for a job that matches poorly), they have a much stronger case for claiming an existing business relationship with you. And once they do that, they can leech off your salary for a job they did not even help you find.