One of the biggest complaints we get from customers is the generic error message "An error occurred on the server. Please try again", that displays in the checkout when their credit card cannot be accepted. Usually there's an issue where the billing and shipping address or zip code doesn't match, so the card is not accepted.
The problem is the error message states there is "an error on the server" which leads them to believe there is something wrong with the website-when that is not the case, there is something wrong with what they've inputted. Another type of generic message like "We could not place this order, please call (store's phone number) to complete" would work infinitely better. Even better, would be a way to customize your own error message in the checkout right in the admin panel.
Overall I think this simple change would help the customer understand that the order could not be processed, but there isn't a problem with the website or server.