Many websites utilize a challenge-response mechanism know as CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) to protect against automating the creation of user accounts, content, or otherwise abusing services they provide. Most common CAPTCHA systems work by generating distorted characters, text, or pictures that can be easily recognized by the human brain but present significant difficulty for computer OCR (optical character recognition) or other image recognition systems.

Enter Social Engineering. Although CAPTCHA may be fairly effective at verifying a reply is from a human and not a computer they do not guarantee that it is from the human for which the challenge is intended. Computer Security Research - McAfee Avert Labs Blog

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