Google Account Signin With CAPTCHA

Google Account login featuring CAPTCHA for additional securityTonight I was presented with a Google login page which was different in a few ways:

  • size and shape of the control were different
  • instead of using an in page control, it took me to a completely new page
  • required additional CAPTCHA validation

I suspect this may have been triggered by logging in and out of various Google products tonight, where I closed a tab but didn’t close the browser, opened new tabs and logged in and out and it might have had conflicting session information.
Does anyone know what causes this type of login prompt to be thrown up by Google?