From Wikipedia-
In the Mahabharata, Shishupala's mother was given a vow by Krishna, her nephew, that he would pardon his cousin Shishupala for a hundred offenses. When Yudhishthira underwent the Rajasuya Yajna, he sent Bhima to obtain the fealty of Shishupala, now king after his father's death. Shishupala accepted Yudhishthira's supremacy with no protest, and was invited to the final ceremony at Indraprastha.
At that event, the Pandavas decided that Krishna would be their honored guest. At that time Shishupala insulted Krishna as a cowherd and worthless to be honoured as a king.[2] At the same event, he committed his 100th sin and was pardoned by Krishna. On insulting him again, which was considered as the 101st sin, Krishna released his Sudarshana Chakra on him and killed him on the spot.