The scenario goes thus:

There is a prediction machine which predicts very accurately but always states its prediction, and a boy who always defy what he is told. If the machine predicts that the boy will turn left, and says "you will turn left", upon hearing this, the boy turns right in defiance.

Because the machine predicts accurately, it knows that if it said "you will turn left", the boy must turn right, and therefore it should predict and say "you will turn right", but if the boy hears that, he will turn left, and so on.