When URL rewriting results in "file not found" errors, systematic troubleshooting is required. First, verify that the rewrite module is enabled in your web server configuration. For Apache, check mod_rewrite is loaded; for Nginx, ensure rewrite directives are properly configured in server blocks.

Examine your rewrite rules for syntax errors and logical issues. Common problems include incorrect regular expressions, missing flags, or improper rule ordering. Test rules incrementally, starting with simple redirects before implementing complex patterns. Use online regex testers to validate patterns.

Check file permissions and paths - rewritten URLs should point to existing files or valid handlers. For single-page applications, ensure your fallback route serves the main application file. Monitor server logs for detailed error information. Implement custom error pages to improve user experience when files aren't found.