One such example that comes to my mind that happens all the time is: a grocery store sends a forecast to the supplier of how much you want to buy, the supplier gets the goods and sends them to you. You cannot sell more than you have =) So the forecast will impact reality, if you actually have two times more customers wanting to buy that product, you will sell only what you forecasted. So 100% accuracy in forecast (because you will sell everything that you forecasted), but in fact, it was a very very bad forecast with 100% accuracy.
One such example that comes to my mind that happens all the time is: a grocery store sends a forecast to the supplier of how much you want to buy, the supplier gets the goods and sends them to you. You cannot sell more than you have =) So the forecast will impact reality, if you actually have two times more customers wanting to buy that product, you will sell only what you forecasted. So 100% accuracy in forecast (because you will sell everything that you forecasted), but in fact, it was a very very bad forecast with 100% accuracy.