What are some common mistakes that can lead to CORS errors in Next.js?

Common mistakes include: Incorrectly configuring the `Access-Control-Allow-Origin` header (e.g., using a wildcard for production), forgetting to set the correct `Access-Control-Allow-Methods` or `Access-Control-Allow-Headers` when using custom methods or headers, not handling preflight requests properly on the backend, and using the wrong origin in your proxy configuration. Also, using relative URLs for API requests when you should use absolute URLs or the environment variables.