If you're going to take the extra time to write a cover letter that you include along with your resume, you might as well write it properly! We talked to a few recruiters and found out that they frequently find mistakes so annoying that cause them to immediately discard some applications all together. Here's a sample of some of the mistakes they mentioned:
Letter addressed to the wrong person or company: It doesn't annoy hiring managers that you're probably applying for other jobs, but it does annoy them when you don't take the time to check that your cover letter is addressed properly. Sending it to the wrong person or company will get your application deleted immediately.
Spelling and/or grammar mistakes: You're probably tired of being told to check and re-check your work, but it is extremely important! When spelling or grammar errors show up on your cover letter, the person reading it is going to think that you either don't know how to write properly or that you didn't bother to check it over. Either way, it's bad news for you.
It's too long: Cover letters should be short and to the point. They should provide some basic information about how you are specifically qualified for the job in question. That's pretty much it. Anything longer than a few paragraphs starts to look more like an essay, and it's an immediate turn-off.
No contact details: It happens quite frequently - people forget to include their name, let alone a way to contact them. While your details may be on your resume, no one wants to take extra time to fish for information that should have been provided for them right away.
No cover letter: This is the worst mistake of all. You're competing against dozens of other applicants who have instantly shown that they took more time to apply than you.
At the end of the day, you just want to give yourself the best chance possible to be called for an interview. Think about what a potential employer wants to know most about you, and try to convert this into a cover letter.