Prepare for smart card debacles. Almost all European credit cards use smart card technology, says Perkins, but since US cards have been slow to catch up, you may find yourself arguing with a merchant over whether he'll accept your card. Don't stress: If the merchant takes Visa or MasterCard, he's obligated to swipe your old school card with the magnetic stripe. The real thing to watch out for are ticket machines at train stations. You may be forced to wait in line for a booth operator to process payment.