When you forced the errors to occur did you go to the checkout failure page as well? If so then the success and failure pages are not mixed up. To determine why you are getting to the failure page with the success message you should add in the full request as well as the full response. This will list all of the information that you are posting to PayPal as well as the information that they are sending back to you.
If you have a hard time determining what the issue is after looking at these details please post back with the full request and response minus any sensitive details so we can take a look at it.
