A typo-filled cover letter is a dead giveaway that you didn’t take the time to proofread or that you simply don’t have a good handle on grammar. Triple check each cover letter before you send it off. If writing isn’t your strong suit, have a friend read it over to make sure it’s perfect.
You should also pay attention to the submission instructions in the job posting. If a posting asks you to send resumes only, don’t send a cover letter! The hiring manager probably won’t read it, and by sending one, you’ve failed to follow directions. Similarly, if a posting asks for you to include a specific phrase in the subject line, copy it word for word. This is a test to see if you can follow directions.
It’s easy for a hiring manager to discard an application because of errors in a cover letter, so don’t let this easily-avoidable mistake cost you an interview!