How do I fix a broken “Agree to Terms and Conditions” checkbox that blocks checkout?

A broken terms checkbox happens when the form validation script fails to register the checked property status of the HTML element, locking customers out of checkout even after they click the box.

“This is one of those problems that almost always comes from a validation script that never actually registers the checkbox as checked. I’ll fix the script and have checkout unblocked within 24 hours for $99.”
— Jeff Bollettino, Managing Director, Ignite Conversions