The following simulation illustrates a Hosted Payment Process Result error during AR invoice payment where a Braintree Connection is primary and a PayPal Connection is the fallback. 

Select an invoice to pay where the Customer Reference is ERR03 and the open amount is >= $3000.

Make a $3000 payment to simulate the Hosted Payment Process error.

Now that the error is simulated you will receive the retry option.

Click retry and the fallback payment form will be presented. Remember that since Paypal is the fallback in this scenario anything over $1000 will decline.  Therefore, this is the end of this simulation.