I've written before about a few different kinds of lies: Convenience, Malice, and Protection. This is another addition to the collection. The entry, as I have it written in my notes, is: "The Lie of Education. You tell somebody a lie, so that as a result they come to realize a truth."
Your friend is having trouble with his girlfriend. They bicker over so many little things every time you see them. Your friend is getting frustrated, and continues to make googly eyes at another, attractive woman. You happen to know that the attractive woman is dumb as a brick and as pleasant as a rabid squirrel. However, you tell your friend that he should go on a break with his girlfriend and go out with the attractive woman. Shortly after following your advice, your friend goes nuts because the attractive woman is so terrible to him. The fights are 10 times worse and about far more insignificant and unintelligible things. Because of you, he has realized that his current girlfriend is actually quite a catch and that the little things can be worked on and are far more tolerable.
That is a lie of education. It is a technique usually reserved for the old and wise. Partly because it is a dangerous one to do. When you tell somebody a lie and send them on their way, you can't guarantee what's going to happen. The best you can do is know the person so thoroughly well, as well as everybody else involved, that you can accurately predict what will happen. Again, usually a skill reserved by the old and wise. But even then, there is no way to account for everything out of your control (other people, acts of nature, mechanical failures, etc.)
The nice thing about the lie of education is that there are a number of ways you can use it. Play it straight and have somebody learn a lesson the hard way. It could be because it was the only way they would learn. It could also be because the liar was just a jerk. You can twist the classic story and make some unintended things occur. Maybe the whole plan falls apart. Maybe somebody learns a lesson even better than the intended one. Maybe somebody learns a lesson completely contrary to the intended one.
As usual, the best examples come from life. Have you ever tried to teach somebody a lesson? Ever try to trick somebody into learning a lesson? Ever have your plan totally explode in your face? If so, write about it. If not, make it up, then write about it.