Some random background noise will be added when the captcha audio is generated to avoid detecting the letters of the code.
In case, where ImageStorageLocation property is set to Session a null reference expection is thrown when CaptchaMaxTimeout expires
Regardless whether the inserted code matches the displayed image or not, the Captcha will take it as not valid. The issue is reproducible in all browsers. video: http://screencast.com/t/JD2XgQXX