My wife got through at SFO with her YMCA membership card a few years ago. She didn't mean to show it, but it was right next to her driver's license in her wallet and she pulled it out by mistake. Not a problem. They didn't even ask if she had anything more official. She realized what had happened when she went to put it back, but was savvy enough to figure out that at that point it was best not to rock the boat.

Of course, the only time this won't work is when it has to work.