Carl's mass-explosive strategy, whose damage prompted the system to make all hunters immune to further explosions.
Carl's bombing campaign against the Hunters worked well enough that it was countermanded outright. Vrah's Note informed him that all hunters were now protected from further explosions, so he shouldn't bother trying to kill anyone else in town with his silly explosives — he'd only be killing his precious NPCs.
The ruling landed with a Wheeled Bomb already prepared and a Clockwork Triplicate delivery plan staged at the saferoom door. Carl scrapped it immediately and apologized to Bomo, who growled unhappily while Sledge went cheerfully back to Frogger Cabinet.
The explosive strategy that dominated Carl's early hunt and was neutralized by rule change.
Also appears in books you've already read — spoiler-safe to their endings: