The document discusses common mistakes that are often found during website audits. It covers 5 categories: content architecture, display architecture, site architecture, security, and performance. Some examples of mistakes mentioned include having similar content types, not reusing fields, extra modules installed that are not useful, reinventing functionality that Drupal already provides well, outdated core/contrib modules, and complex queries without indexes. The document provides best practices for each category such as planning content architecture ahead of time, separating logic from presentation, using the right hooks for custom modules, keeping software updated, and optimizing databases before caching. It emphasizes the importance of testing, environments, and maintenance for the website lifecycle.