Steps to reproduce:

• Switch to QA CAPTCHA (probably happens with all CAPTCHA plugins).
• Make enough failed login attempts to get the CAPTCHA displayed.
• Enter the correct username, incorrect password, and correct CAPTCHA code and submit.
• Login fails. The CAPTCHA is no longer displayed even though the login error clearly states that the CAPTCHA code must be provided.
• Provide the correct credentials and submit.
• Login fails because the nonexistent CAPTCHA code was not provided.