In his death-bed Bhishmadev lovingly recollects the situation in the battlefield when he had made a promise to Duryodhana that he will kill Arjuna until Krishna comes in the way taking the weapon. He also assures Duryodhana that Krishna has also made a promise of not taking weapon in the battle so being truthfulness personified (satya vrata), He will not take weapon and hence there is a chance for Bhishmadev to kill Arjuna. On that day Bhishmadev fought so vigorously that Arjuna could not withstand the barrage of arrows that were flying towards him. There was a point when Arjuna and Bhishmadev were so close face to face and Bhishmadev was about to shoot the arrow to kill Arjuna. At that point, the most merciful Lord, in order to protect His devotees (both Arjuna and Bhishmadev) Himself got down from the chariot, took up the wheel of the chariot and rushed towards Bhishmadev in an angry mood to kill him. Bhishmadev dropped his weapons and was thrilled to see the Lord coming towards him in an angry mood. Bhishmadev thus prays beautifully in SB 1.9.37:
sva-nigamam apahaaya mat-pratijnaam
rtam adhikartum avapluto rathasthah
dhrta-ratha-carano 'bhyayaac caladgur
harir iva hantum ibham gatottariyah
"Fulfilling my desire and sacrificing His own promise, He got down from the chariot, took up its wheel, and ran towards me hurriedly, just as a lion goes to kill an elephant. He even dropped His outer garment on the way."
Srila Prabhupada says beautifully in the purport, "Because Bhishmadeva wanted to see Lord Krishna take up some weapon to save His friend, the Lord created this situation, making Arjuna's death imminent. He stood before Bhishmadeva to show him that his promise was fulfilled and that He had taken up the wheel."
This is the beauty of the Supreme Lord. To save His devotees, He will go to any extent. However this is not the only time He broke His own promise against Bhishmadev. From HH Gopal Krishna Goswami Maharaj's transcendental diaries we found another occasion where the Supreme Lord broke His own promise to save the promise of Bhishmadev. Maharaj has written this in hindi but we will try to paraphrase within our capacity:
"When Bhishmadev did not agree to marry Amba, Amba went to his spiritual master Lord Parashurama and asked him to order his disciple to accept her. However Bhishmadev was not willing to accept. So a tremendous fight ensued between Parashurama and Bhishmadev. Bhishmadev made a promise that if he loses this battle, then he is not the son of Shantanu and Ganga. When the terrific fight was going on, at one point Parashurama was about to release His axe weapon at Bhishmadev, the weapon which He used to kill so many generations of kings. But realizing that Bhishmadev's promise will be broken if He killed him, He refrained from using His axe weapon."