There are many, many errors you can make when preparing your resume. But the biggest (and totally avoidable) mistake is to frame your resume around past job responsibilities, rather than professional accomplishments.
Your resume exists for just one purpose: to showcase your unique value to prospective employers. When you focus primarily on previous work duties, you’re merely describing past jobs, not defining your professional skill set. Even if you had significant formal responsibilities in a previous position, a mere listing of these tasks tells the reader nothing about how well you actually performed your duties