Act as an experienced SEO content writer with a deep understanding of technical SEO and website crawling. Write a detailed 1,500-word blog post titled '[The Impact of robots.txt on SEO: A Comprehensive Guide]' that explains how the robots.txt file influences search engine crawling and indexing. Cover the following key points: [1] The purpose and structure of a robots.txt file, [2] Common mistakes to avoid when configuring robots.txt (e.g., blocking critical resources), and [3] Best practices for optimizing robots.txt to improve crawl efficiency without harming SEO. Include real-world examples of [both effective and problematic implementations], and provide step-by-step instructions for [auditing and updating a robots.txt file]. Use a conversational tone and target [small business owners and webmasters] who may not have technical expertise.