Spelling and Grammar - A simple but much overlooked detail.  These errors are the tell-tale sign of carelessness and laziness. Make sure you have your resume proof read for spelling and grammar errors before sending it out.  An independent third party can see mistakes you've overlooked several times.  Make sure you watch your abbreviations and avoid slang and excessive use of duplicate words.

Too Long - Employers read a lot of resumes. Keep to the point. Resumes that are too long are probably filled with non-pertinent information.  Stays focused and keep it to one page.  You don't need to write complete sentences or paragraphs.

Difficult Reading -  Watch your font type, paper color, asterisks, and bullets. Make sure it flows and is easy to read.  The reader should enjoy reading your resume, not struggling through it.   Try and say as much possible with as few words as possible.

Job Specific - If your resume is too generic it will appear that you'll settle for any job available. You're trying to make a good fit to a specific position within a company.  One that you're best qualified for over the other candidates. In order to accomplish this, you need to be specific in your objections and accomplishments.
Including 

Non Essential  Information - Omit your personal information like marriage status, children, religion, height, weight, etc. 

Too Modest - Your resume is a sales tool. Be bold and assertive without being arrogant or supercilious. Make sure your resume is factual and truthful. There's a lot of competition with other applicants so make sure you resume shines!

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These are little things that mean so much, worth making necessary attempt to avoid in order to increase chances of being employed.