An Incident to Reflect Upon.
Hadhrat Ibnul Hasan, may Allah have mercy on Him, once remarked: " I am grieving for forty years over a sin commited by me." Someone enquired: "What sin is that which has caused such great concern and greif in you?" He replied: "once I was visited by a friend and in order to entertain Him, I bought a fish which we later ate. After the meal, I scraped some sand from my neighbour's wall without his permission to wash my Hands. I am still crying over that sin."
The questioner expressed surprise over the fact that such a small fault has created so much anguish and concern. So Ibnul Hasan explained that according to the prophet peace be upon Him, the biggest sin before Allah Ta'ala is that little slip/error which man regards as trivial and unimportant and therefore thinks it unnecessary to seek forgivness.
On the other hand, if He acknowledges His wrongs, considering them quite serious, he would certainly feel guilty and ashamed of His actions and this would lead hHim to repent, seek Allah Ta'ala's forgivness through which he receives pardon and in this way obliterates all trace of the sin.