A couple days ago I found an ebay seller with cards in stock and tried to buy 5 of them, but because it was a large purchase I got an automated call from my bank asking if I had authorized the transactions. I didn't get to my phone fast enough to answer, but I listened to the voicemail, called back right away, and confirmed that I had authorized it.
Then I checked ebay and it said that my order still wasn't paid. I checked PayPal (which I paid through using my debit card) and the transaction wasn't there at all. I checked my bank account and the transaction wasn't there either!
I contacted the seller and asked if they received the payment and then got a message a few hours later saying that they had no record of the order at all! Worst of all, by this time they were all sold out!
That experience was a prime example of the type of problems that cryptocurrency is going to eliminate.