I've never had a problem with unknown charges or the like showing up except for one time when I was staying at a Hyatt they charged me for a free room and I didn't find out until I got my credit card bill, but the hotel quickly fixed that problem.

The one problem I used to have with my AMEX green card is that when I was traveling a lot and charging a lot in different states (car trips) they would start rejecting my credit card and I would have to call customer service and let them know my card wasn't stolen, etc., and the problem would go away ... but it was still a pain.

Now that I have a Platinum personal and green corporate card I don't have that problem anymore.