Yudishtra lying about Drona's son dying to get Drona to drop his arms.

Actual death of Bhishma which left a strategic hole in the Kaurava strategy

Karna's death through guile by Krishna and Arjuna

Kunti secretly met her abandoned son Karna and requested him to spare the Pandavas, as they were his younger brothers. Karna promised Kunti that he would spare them except for Arjuna.