How can I debug file upload scripts to identify the cause of a 500 error?

Use PHP's error reporting features (e.g., `error_reporting(E_ALL);` and `ini_set('display_errors', 1);`) to display errors during script execution. Add logging to your code to record file upload attempts, including any errors encountered. Use a debugger to step through the code and identify the point where the error occurs. Validate input data thoroughly to prevent issues caused by unexpected file types or sizes.