Typos and Grammatical Errors
Your resume needs to be grammatically perfect. If it isn't, employers will read between the lines and draw not-so-flattering conclusions about you, like: "This person can't write," or "This person obviously doesn't care."
Always take the extra time at the end of updating your resume to run spell check again. This has saved me many a time because if you have all the correct letters in a word but not in the correct order your mind will correct it for you when you speed read it and you wont see the mistake.