Here are some simple but critical things a developer must never over look: -
1. Using Non-Responsive Design
2. Depending on Website Builder Tool
3. Using Outdated HTML
4. Insufficient Input Validation
5. Cross Browser Compatibility Issues
Other common mistakes like poor content, security issues, poor design, wrong SEO(Search Engine Optimization) techniques, and many others. All of which may lead to your website placed lower in the search engine results.