PayPal's PayFlow Pro product would return valid for 1XX, 2XX, 3XX CSCs and different errors (mostly CSC verification related) for 4XX, 5XX, 6XX, 7XX, 8XX, 9XX. Was quite useful for testing.
I think you just pointed out about the only useful thing out of the entire PayPal integration experience.
I'd love to know how it got through their formal Make Developers Hate Us processes.
I tried with all ones (after the card #) and it would not accept the expiry date. I admit, I might be part of the problem. ;)