When we use physical cues placed in our environment as a mechanism for coordination, this is known as stigmergy. I think most of us do this even with ourselves (autostigmergy), e.g. placing our car keys on top of an object we need to remember to bring to the office, or leaving an important document on top of our laptop as a reminder to fill out and mail the paperwork in the morning. And yes, stigmergy (rather than explicit communication) is often involved in biological or social state machines. It’s rather delightful.
This story has the hacker ethos to it; the author not only found and documented an FSM, but embraced being part of the state of the machine and changing its behavior from within.
I have been in a restaurant once where they placed the beer coasters directly after ordering before the drinks were served. My assumption is to track the state of “has ordered”.
It’s a great system. The place near me that uses it handles another state: Partial delivery of order. Before removing the number, they ask if you’re waiting for anything else.
Of course all systems fail too, and an important measure of a system is how elegantly it deals with failure. I’ve only ever seen this system fail in ways that don’t negatively affect the customer: