Krishna met Bhishma to offer respect when he was lying on the bed of arrows. .
Bhishma asked Krishna “Hey Madhav!, it is said that earth is the land of Karma, where we receive results of our past good and bad deed. I remember all my past 17 births, I don’t remember any sin committed by me in these births, then why I have fallen like this in bed of arrows?”.
Lord Krishna said, in your 18th birth in the past, you had pierced a snake with an arrow and threw it in a place which was full of thorns. Many thorns pierced through his body, his family members came to rescue him, but was helpless. Before dying the snake cursed you, “I am unaware of your identity oh man, you will die one day the same way I die, you too will have a huge family, but nobody will be able to help you, no one will be able to remove the thorns from your body, you will die out of pain, helpless like me.”
Krishna said ” You had followed the path of bakthi (devotion) in all your past 17th births, so were protected from the sin you had committed. But now, you have selected to fight on the side of Adharama, You have fell dowbn from the good position karma had given you by fighting on the unrighteous side. Hence the sin has affected you !!