A bad habit in itself does not mean that I have done something; but if this bad habit leads to an action, this action changes the external world. In fact, everything which thus exercises an influence upon the physical world returns to us during our next life as our external destiny in the physical world. Thus the deeds of our physical body during this life become our destiny in the next. We learn this through being placed in this or in that life-situation. Whether a person is happy or unhappy in one or other condition of life depends upon his actions during his preceding life.
Source: Rudolf Steiner – GA 100 – Theosophy and Rosicrucianism: Lecture VII: The Law of Karma – Kassel, 22nd June 1907