Most CAPTCHA-solving services, such as 9kw.eu, offer the ability to report issues with solved CAPTCHAs and give feedback as to whether the text worked properly on the site or not. You may have already noticed that we have a report method on our CaptchaAPI class, which allows us to pass the CAPTCHA ID along with a boolean to determine whether the CAPTCHA was correct or not. It will then send the data to an endpoint used just for reporting CAPTCHA correctness. For our use case, we can determine if the CAPTCHA was correct by determining if our registration form succeeds or fails. 

Depending on what API you use, you may get returned credits when you report incorrect CAPTCHAs, which is useful if you are paying for the service. Of course, this could also be abused, so there is usually an upper limit on error reports for each day. Regardless of the return, reporting both correct and incorrect CAPTCHA solutions can help improve the service and allow you to not pay extra for invalid solutions.