This is an interesting story i copied from HERE talking about how powerful istighfar can be!
This story about “Istighfar” is from the life of Imam Ahmed Bin Hanbal, who is considered as a renowned scholar of Islam and a famous theologian. During his old age, while Imam Ahmed was travelling he stopped by a town. After the prayers, he wanted to stay for the night in the masjid yard because he didn’t know anyone in the town. Owing to his humility, he hadn’t introduced himself to anyone thinking that if he did, he would be welcomed by many people.