If I install reCAPTCHA and set it to be used by a form but I do not configure reCAPTCHA, any page which should contain the reCAPTCHA is blank except for this message:
To use reCAPTCHA you must get an API key from https://www.google.com/recaptcha/admin/create
This has the potential to completely break a web site. If reCAPTCHA is not configured, Drupal should behave as if there is no CAPTCHA configured or perhaps fall-back to a default CAPTCHA.
(I wonder what happens if reCAPTCHA is properly configured, but the reCAPTCHA servers go down or are inaccessible. Does that also trigger a similar problem?)