Dhuryodhana's promise to Arjuna : Little known tale.
Dhuryodhana was once attacked by a Gandharva and taken into custody. Upon learning this Yudhishtira sent Arjuna to rescue Dhuryodhana as he was their brother. Arjuna rescued Dhuryodhana successfully. Upon this Dhuryodhana expressed his gratitude and told him that in future Arjuna could ask anything from him and it will be given.
Years passed and eventually the Kurukshtra war started. Arjuna had even forgotten about the promise. On the first day of the war Drona and Bhisma decimated Pandava army killing thousands of their soldiers. It was a victory for Kauravas on day 1. Sensing the challenge posed by Bhishma and Drona, Pandavas created a day 2 strategy that involved keeping Bhishma and Drona away from the main battlefield. This paved way for Pandava victory. Dhuryodhana was a furious to learn this. He barged into the tent of Bhushma and accused him of not fighting to his full potential. Bhishma was angered by these words and used his yogic powers to materialize 5 gold arrows. Bhishma vowed to kill all Pandavas using these 5 special arrows. Dhuryodhana was not willing to trust Bhishma and demanded that he would take the arrows in his custody for safekeeping.
That night Krishna's spies passed this information to him. Krishna went to Arjuna and reminded him of the debt Dhuryodhana owes to Arjuna. Arjuna though hesitant went to Dhuryodhana and demanded those 5 arrows and destroyed them. Thus Bhishma was not able to kill them on the 3rd day of the war.
Even though Dhuryodhana was born in a noble house and was strong, he lacked the mental strength to trust others and make sound judgement. He should have trusted Bhishma, should have exercised caution in giving promise to Arjuna but in his anger he made all the mistakes that caused his downfall.