You are the central bank. The short-run Phillips curve offers a bargain: push inflation above what people expect and, for a moment, unemployment drops below its natural rate, u = un − β(π − πe). Only the surprise buys the discount.
But expectations learn. Each quarter people revise what they expect toward what they just saw, πe → πe + λ(π − πe). As they catch up, the short-run curve slides up under you and unemployment drifts back to un.
So to hold unemployment below natural you must keep inflating faster than expectations — quarter after quarter, at an ever-rising rate. That is Friedman and Phelps' accelerationist hypothesis: there is no permanent low-unemployment bargain, only an accelerating price.
Reach the natural rate at 2% inflation, then at 9%, and you have drawn the punchline: in the long run the Phillips curve is vertical. Unemployment always returns to un — the economy chooses the inflation rate, never the unemployment rate.
Something in the simulation stopped unexpectedly — the lesson continues without it. You can move on; nothing you did was wrong.