If you are using BluePay as your online giving vendor, you may run into a situation where certain card types get rejected during an event payment process, or during the giving process. (We've seen this a good deal with Discover Cards).
BluePay has indicated that sometimes if you turn on the CVV2 Filter in your BluePay portal, this can cause mismatches to void the transaction. The best thing to do here is to log into your BluePay portal and turn off any CVVC filter.
If you remove this, then walk through the event payment form or giving form, the transaction should go through without any issues.