Testing his new suit, Tony climbs too high and the armor ices up and stops working, a weakness he learns about the hard way.
Tony leads Stane's copied armor high into the sky, where it ices up: the same flaw Tony ran into on his test flight.
The setup and payoff sit in the same film. According to the film's production history, the ending was rewritten late to call back to the freezing flaw because the first cut of the final act was just two robots punching each other.