To automate the testing of some functions of the server that allows user input it could be needed to bypass a captcha implementation. Test these things:
Do not send the parameter related to the captcha.
Change from POST to GET or other HTTP Verbs
Change to JSON or from JSON
Send the captcha parameter empty.
Check if the value of the captcha is in the source code of the page.
Check if the value is inside a cookie.
Try to use an old captcha value
Check if you can use the same captcha value several times with the same or different sessionID.
If the captcha consists on a mathematical operation try to automate the calculation.
If the captcha consists on read characters from an image, check manually or with code how many images are being used and if only a few images are being used, detect them by MD5.
Use an OCR (https://github.com/tesseract-ocr/tesseract).