That's what I thought too but it wasn't working. The error was actually the checkout_failure.php page showing after I tried to submit my credit card. The checkout uses an iframe on the site. The payment form started to submit then the failure message showed. This means that PayPal returned an error in the response right?
I will try it again in case for some reason I had a syntax error when trying it before. I don't think I did but you never know.