What happens when a block of silver is left in the open! After some time it develops a black coating. Even though the entire block is pure silver but on the outside is the dirt. Similar is the case of a self realized person. Once a human being reaches state of self realization, it remains permanent forever.
But due to existence of manifest physical body and laws of karma one accumulates karma, dross impurity but only externally as in the case of silver block. This external impurity can be easily cleansed by a self realized soul at will.
Karma negates to zero only when a self realized soul leaves the mortal frame forever. Otherwise small traces of external dross impurity remained in the self realized person.
After reaching state of self realization I went to my office that I had already abdicated to my brother. In the elevator my thought process became so pure that my legs started coiling like a snake and I felt my soul atman leaving the body forever.
Immediately I invoked an impure thought just to retain my body and I was back again. Thereafter, I always keep a check not to become so pure that my body untimely leaves me. I still have some family responsibilities and after completing those I shall become free to leave my mortal frame.