validate is failing on a field that is not a required field

validate is failing on a field that is not a required field

I have a credit card field that is only mandatory if they select a credit card as a payment type.

If they select Gift Certificate, then I set the card to not be mandatory:

$('#creditcard').removeClass('required').removeClass('creditcard');
validationRules.rules.CardNumber.required = false;

however the validation still fails on the creditcard field. If I enter this in  to Firefox's console:

$("#OForm").validate().element("#creditcard");

it returns false.

Hopefully someone can give me some additional troubleshooting tips.

To make matters more confusing, this validation only fails on my form when I am passing a particular parameter in the URL string. I've scoured my code and cannot figure out why that parameter would have any impact whatsoever on this matter.