Most writings on the doctrine of karma emphasize the strict lawfulness governing karmic actions, ensuring a close correspondence between our deeds and their fruits. While this emphasis is perfectly in place, there is another side to the working of karma - a side rarely noted, but so important that it deserves to be stressed and discussed as an explicit theme in itself. This is the modifiability of karma, the fact the lawfulness which governs karma does not operate with mechanical rigidity but allows for a considerable wide range of modifications in the ripening of the fruit.
If karmic action were always to bear fruits of invariably the same magnitude, and if the modification or annulment of karma result were excluded, liberation from the samsaric cycle of suffering would be impossible; the inexhaustible past would ever throw up new obstructive results of unwholesome karma.
-- Nyanaponika Thera, The Heart of Buddhist Meditation