After Yom Kippur, the normal night prayer service includes a prayer for the forgiveness of sins. Our rabbis ask, “Isn’t this a blessing in vain?” Because these night prayers are immediately after Yom Kippur, what sin could we be repenting?
The sin that most Jews commit right after Yom Kippur is the belief that they are not truly forgiven. They believe that the problems of the year before have “survived” the prayers and the day and continue to exist in the day after. This is a grave sin, as you are not believing in your own prayers and untrusting of Hashem for releasing you from your sins.
We must be confident that our prayers and thoughts were heard by G-d and to believe that we are now clean from our sins after Yom Kippur, our Day of Atonement.