object lesson 
noun
A striking practical example of a principle or ideal: they responded to daily emergencies in a way that was an object lesson to us all...
- Theirs is an object lesson in making good in the face of adversity and it has been a privilege to represent them.
- If those right shoes do not arrive, and for that matter it is not clear what he will do with them, all concerned will have had an object lesson in the fact that two lefts do not make a right.
- There's no question about that, but you know, this has to be an object lesson for parents who are sending their children off.