The most common mistakes we see on self-written resumes are errors in mechanics. Misspellings, punctuation goofs, syntax errors, and spacing issues are common with misplaced hyphens scoring highest for most common mistake. Having an error-free resume is important because errors speak volumes to the reader. Mistakes in a resume tell the reader you do not pay attention to detail, you are a poor marketer, you are sloppy in presentation, and can even intimate lack of education. These are all messages you want to avoid giving in a resume.
Most people who do not write for a living rely on spell-check. That's a huge mistake in-and-of itself. Spell-check is handy but it is not as thorough as it should be. For example, spell-check doesn't show manger as a misspelling because it isn't, even if you meant to type manager. Always make sure your proofread the old fashioned way using your eyes. Here are a few tips we pro's use:
Print it out to proofread. Often, it is easier to see errors when printed on paper than when they just appear on the screen.
Read it backwards. Start at the end of the document and read right-to-left. This compels you to look at every word individually and negates reading for meaning.
Use a ruler. Read every line individually by using a ruler. I have a clear plastic ruler that has a highlighted area in the middle (I think it was made for accountants) that allows me to highlight each line and read it slowly.
Get new eyes. When you have worked on a document (any document) for a long period of time, you no longer see what you have written but rather see what you THINK you have written. Getting someone else to read it for you who has never read it before can help spot areas where syntax is off or you have other mechanical errors.
Hire a pro. Many people just aren't gifted in writing. Others are. Outsource the entire project to the pro's and save yourself hours of struggling with a resume in which you still do not have confidence in the end.
Read all numbers three times for accuracy. A wrong phone number can kill your job search.