This study reveals the frequency of technical and on page SEO mistakes appearance. Spoiler - we all making SEO mistakes!
https://www.semrush.com/blog/technical-seo-mistakes-semrush-study-2017/
* SEMrush Site Audit tool * the same study last year * 13 Most Common Hreflang Mistakes - SEMrush Study * 10 HTTPS Implementation Mistakes - SEMrush Study * The Ultimate Site Audit with SEMrush