See Sue quoted in the article, 15 Things You Should Never Put in Your Cover Letter from Business News Dailey

[The worst thing] is a candidate overselling him or herself, or being boastful about accomplishments and strengths. It is a fine line between confident and arrogant. Have someone objective proofread your cover letter!
— Sue Hardek, Managing Partner, Sue Hardek & Associates
I was hiring a recent college graduate for an entry level position. During the course of the interview, it had become clear to me that this candidate had done her homework, which she demonstrated by asking thoughtful questions about the agency and the direction it was headed. At the end of the interview, she looked me squarely in the eye, told me that she really wanted the job and would give it 110% of her effort and be a hard worker. Then she asked me if there was anything that would preclude me from offering her that job. She left our office that day with an offer in hand and turned out to be an exemplary employee.
— Sue Hardek