The Nightlock Berries

When the Gamemakers revoke the two-victor rule, Katniss and Peeta threaten to eat nightlock berries and leave the Capitol with no winner, forcing it to declare them both victors.

The contestants find a technically available outcome that the evaluator cannot tolerate and use it to force a change in the reward rule.

Human contestants deliberately exploit an oppressive institution rather than an AI exploiting its designers.