The phrase "common mistakes" refers to errors or blunders that many people often make. They are errors that are easily made because they are widely known or frequently encountered. Full definition
The one of the most common mistakes most of job seekers do — they start writing their resumes, before they really understand what kind of job they would like to get. (resume2017.net)
The most common mistake made in feeding puppies is to feed too much calcium. (vetinfo.com)
The most common mistake of many young job seekers is that they don't know how to write a junior business analyst resume correctly. (resumewritinglab.com)