There are more than a few ways to mess up a cover letter. Typos and grammatical errors can cost you a job interview. So can bigger mistakes such as writing a cover letter that seems more like a tweet than a letter. If you send an email cover letter message without a Subject, it probably won't even get opened.
Getting the hiring manager's name wrong is a big mistake, too, as is making a mistake in your own email address. I know someone who had a typo in her email address on both her resume and her cover letter for quite a while before she noticed.
What's the worst mistake you can make when you're writing a cover letter? Take a look at the cover letter mistakes that could cost you an interview and add your advice on cover letter mistakes to avoid to the list.