What is the difference between acting by ignorance and acting in ignorance?

For Aristotle, acting by ignorance means that one is acting in such and such a way because she wrongly thinks that it is a good course of action. However, it is possible to act in ignorance without acting by ignorance. Aristotle cites the example of a drunk person, who is acting in ignorance in the sense that he doesn’t know what he is doing, but is not acting by ignorance because there is actually something else causing his actions.