State

My smart phone has 2 great features. When I listen to something with my head phones, and I pull the head phones out of the cinch plug, the music or the podcast stops playing until the headphones are plugged in again. The other is that when I get a call while listening to music over my headphones, the music is interrupted, until the call is over.

What could go wrong? Today I was listening to music and got a call. The music stopped playing and I picked up the call. When I hung up, the music started to play again, this time however on the speakers. So there I was, quickly trying to turn down the music.

This little real-world example shows how hard it is for developers to work with state, by itsself a state can be pretty simple, but introducing another degree of freedom increases complexity by a magnitude.