When a customer processes a free order (either $0.00 price or result of a coupon code), there are references on the receipt page and email receipt to the Credit Card on file. Though the card was not used for this particular transaction, it still lists the card information. I'm not so sure this is a huge issue, but may be confusing for $0.00 orders where no card should be involved...
NOTE: If the customer account already has a card on file, the behavior is different than if the customer is New or not entered a credit card yet.
The following is displayed on the Receipt / E-mail:
Card Number: (Visa)
I'm not sure why "Visa" shows up here, perhaps a fallback option? Further, no card is listed since no card exists on file.
A possible remedy to avoid confusion for both circumstances may be to prevent showing this "Card Number" line item for $0.00 order totals, or at least when no card exists.
I thought this was a new bug during my v1.0 testing, but it appears to be present at least in v0.7.2. (I have not tested v1.1)