Fill It With Mistakes
Who cares if you have a few typos, misspellings and errors in your cover letter? Potential employers, that's who. Not only will hiring managers notice your grammatical missteps, they'll view mistakes as a sign of carelessness and poor writing skills. In fact, many hiring managers immediately toss or delete resumes with error-laden cover letters. It's an easy way to quickly cull a pool of applicants.
Some of the most common typographical errors are misspelling the hiring manager's name or title; applying for one position, but mentioning a different one in your cover letter; and inserting the wrong company name in your letter [source: Work Source].
To avoid these errors, run your cover letter through spell-check to catch plain old misspelled words. Next, proofread it yourself, as spell-check doesn't catch things like writing "sea" instead of "see," and won't call you on misspelled names or incorrect position titles. Finally, have a trusted friend who's a good speller and writer read it one more time.