Some of the most common on-page SEO mistakes include keyword stuffing, poorly written meta titles and descriptions, thin or low-quality content, missing heading structure, broken internal links, slow page speed, and ignoring mobile optimization. These issues can hurt both search rankings and user experience, so they can be avoided by focusing on high-quality content, using keywords naturally, maintaining a clear page structure, regularly checking technical issues, and ensuring pages are fast, accessible, and user-friendly.