Blanket immunity granted to all hunters after Carl's mass bombing, nullifying his entire explosives strategy.
Vrah's note informed Carl that all Hunters were now protected from further explosions, so any bomb he set off in town would only kill his precious NPCs. The change was clearly a response to Carl's Doomsday Scenario and the scale of the damage his bombs had already done.
It landed at the worst moment: Carl had a wheeled bomb prepared and a Clockwork Triplicate delivery plan ready to roll out the saferoom door. He scrapped it immediately, apologizing to Bomo and promising to save the bomb for another time.
The rule change that strips Carl of his signature weapon minutes before the hunt begins.