The credit card one is especially important. We run a hosted online ticket system for some of our customers and noticed they were getting a bunch of 'Invalid CC' responses from their payment gateway (and paying for each invalid attempt).
We implemented the Luhn[1] algo credit card check on the checkout page. Invalid CCs would trigger a little warning but still allow the form to be submitted. Invalid CC transactions dropped ~90% immediately. Even better we were able to get rid of the 'select your card type' field since that was detected by Luhn. A little JS was a win all around.
We implemented the Luhn[1] algo credit card check on the checkout page. Invalid CCs would trigger a little warning but still allow the form to be submitted. Invalid CC transactions dropped ~90% immediately. Even better we were able to get rid of the 'select your card type' field since that was detected by Luhn. A little JS was a win all around.
[1] http://en.wikipedia.org/wiki/Luhn_algorithm