I know it's been around for a while, but I was just recently in a chatroom-type-thing, and we played the "Hit Alt+F4 to change your font color" trick on one guy. Dude probably closed his browser a dozen times before he gave up. Dude kept coming back.
Nope. It worked just fine. It's just because it's your account that you can see it. Cookies, and all that. I can't tell at all that you've got a Diner's Club card.
edit: Actually, that number doesn't make sense. 34XX is an american Express card, but then you have too many digits. Amex cards are only 15 numbers.