The cool thing is if you have a card that sends push notifications.  Because they go through your bank they often arrive faster at your phone than the transaction settles on the terminal.  I had a magical moment where I saw a charge on my card before the ticket machine saw it, and then it failed to print a ticket because it was out of paper and before the error came up, i already got a push notification that the money was reimbursed.  Networks are fast.