I was able to reproduce the checkout failure. Not sure what was different this time but the checkout failed. But more than that, the checkout_failure.php page loaded inside the iframe instead of on it's own. Normally, if the checkout fails, the failure page loads on it's own. The $errorURL uses the full URL for the checkout_failure.php page.
I did capture the sessions though by opening the iframe into a new tab and since I had set it to dump the sessions, I have them. What should I look for in terms of the reason it failed?.
Any thoughts on why it would load the pages inside the iframe instead of redirecting to the actual page?