One who becomes aware of a sinful act committed can atone for that through the prayschitta prapatti. This is done at the ripe old age, when one suffers due to several sins and to remove the effects of those sins (to reduce the physical suffering).
This can be done if required. That is, when we realize that we have committed several sins as a result of which we suffer, we can perform prayschitta prapatti as an atonement.