What are common causes of the reCAPTCHA error message "bad request"?

A "bad request" error with reCAPTCHA usually indicates an issue with the format or parameters of the request being sent to Google's reCAPTCHA verification API. Common causes include: incorrect Content-Type header (should be application/x-www-form-urlencoded), missing or malformed POST parameters (like 'secret' or 'response'), or sending the request via GET instead of POST. Carefully review your server-side code to ensure the request is correctly formatted and uses the proper HTTP method.