The iOS #calculator meme that’s going around where “50+50×2=” yields 150 and not 200, is a great example of hidden states in UX design.
Low-cost desktop calculators perform (most) operations strictly left-to-right: press any operator button (+-×÷), and the display is updated to show the result of the calculation so far, and that result becomes an operand for the operation. There is no other state “inside” the calculator that affects the solution—what you see is all you get.
On the iOS calculator, you’re entering the entire expression into a hidden buffer. You can’t really see what you entered (and there’s no option to make it visible, as far as I can tell), and the final result is only shown when you press =. The confusion arises because a user’s mental model may not correspond to the hidden model used for the computation.
All this is to say that hidden states often break #UX expectations. Making hidden state visible goes a long way to remove confusion (see how PCalc does it in the second image).
