$0 Transaction When Customer Bought Download Credits

I just had a customer transaction come through for $0 (no accompanying Paypal transaction).  The transaction was for 4 download credits for $20.  Order log shows the correct item purchased (4 download credits for $20), but shows a $0 price tag.  

I did recently adjust the options for purchasing download credits (it was at one point 5 for $25).  What happens if something is in someone's cart then that item is no longer available?  Not super worried about this since it's just $20 and isn't anything tangible, but just trying to figure out what happened here.

No coupons in play.  There are DL credit redemption codes in the wild, but none were redeemed by this customer.  The customer created the account just minutes before the transaction, but the IP address visited the site many time prior to that. 

It is possible changing up the price changed the product ID and could have possibly caused that with existing carts. Not positive though. I can't think of any reason for that to happen.