Many moons ago (late 90s) I got charged for Skiing Apparel bought in Aspen Colorado (about 700 bucks worth) I called Amex and told them Not only that I did not Buy anything in Aspen but I don't Ski and I've never been there. (at the time) They assured me the Charges were valid because The Store had swiped my card.
I told her OK then could she check the charge that was right before it and if they had swiped the card also. She told me it had been swiped also. It was a charge for a restaurant in Columbus Ohio and according to the time stamps on both charges I ate Lunch at around Noon in Columbus Ohio then somehow was in Aspen 18 minutes later to buy 700 bucks worth of Ski Clothes.
Her manager fixed the charge and waived the yearly fee for my Amex Gold for the Next Year and sent me a new card.
Another time, MC called me to say that there had been a series of small ($1) charges on my card to iTunes. When I said that I didn't use iTunes, they immediately cancelled my card and reissued another. They told me that a lot of people test fraudulent cards by using them for very small purchases, then use them for bigger ones once they know they work.
Overall, I have had good experiences.