I wrote about an experience with this last January, but Bruce Schneier just blurbed about it on his blog. The examples he provided are far more obvious.
It reminded me of something else I saw at an airport a few years ago. (I'm not going to name the airport or the airline. And please don't let this affect the faith you have in the rest of the security theater associated with air travel.) When passengers are not getting on/off the plane the doors to the gateways taking you to the planes are locked and must be opened by a cipher lock. I watched as a pilot walked up the a door and clearly punched 1-2-3-4.
When you think about it they have to make it something that they all can remember, right?