I've been thinking about elevator algorithms lately too. My building applies the principle of least privilege to the elevator - residents are issued fobs which

I've been thinking about elevator algorithms lately too. My building applies the principle of least privilege to the elevator - residents are issued fobs which give them access to the floor where they live, plus any common areas. The problem is that waving the fob in front of the sensor pad before pressing the button for your floor takes a bit of time, and in that time, it's possible that someone else, somewhere in the building, will press the button to call the elevator. When that happens, the elevator may change it's direction. A race condition! For example, you're on the 10th floor and want to go to the first floor. You press the down button. The elevator arrives, with it's "going down" arrow lit. You get on, and as you fumble with your fob, someone on the 18th floor presses the down button. The elevator now has a call on the 18th floor, and no destinations yet, because you haven't pressed the "1" button. Now you have a choice: press "1" and ride the elevator up to 18, knowing that it'll (eventually) go down to 1, or get off the elevator press "down" in the hope that another elevator will arrive before this one gets back. Of course, it's very likely that you don't notice the "going up" arrow, because you're fumbling with your fob. I'm finding this happens surprisingly often, and I think it's a bug.