I guess my funniest story of being carded was from two years ago, when I was in Atlanta for Dragon*Con. I was in the Hyatt's bar, and ordered a Guinness. The bartender asked for my ID. Now, I don't have a driver's license, and my passport was in my hotel room (why would I carry it around with me when I didn't need to?), so I pulled out my Ontario health card, which has a picture and my birthdate. It's not legal to use it for proof of age here, but in Atlanta, they didn't know that.
So she took it from me, looked at me, looked at the picture, looked at my birthdate, and started to hand it back. Her eyes suddenly got wide, and she pulled it back and looked at it again. She then handed it to me and said, "I'm sorry." (I was 41 at the time.)