an object lesson

an ˈobject lesson

a practical example of what you should or should not do in a particular situation: It was an object lesson in how not to make a speech. He did absolutely everything wrong.An object lesson was a school lesson that used real objects as a way of teaching in a very direct and practical way.
See also: lesson, object
References in classic literature ?
The collection, I assure you, was an object lesson in human malignity.
All this is used up; it is no longer instructive as an object lesson in revolutionary anarchism.
While Spotlight (see above) is an object lesson in how to tell a true story, here's the polar opposite.
Duxbury cites Kilmuir as an object lesson in why one should be careful about labeling people.
During his speech, yhsanoy-lu said Medymak Hotel should be turned into a museum as an object lesson for the people.
Heavy conditions didn't help but there was an overload of errors by both sides in a game that never really took off but at half-time, Ponty led 19-0 and it seemed business as usual for the home side Early on they gave the massive Swansea forwards an object lesson in the dark arts of the breakdowns and were well worth their lead at the interval with the All Whites barely getting a look in.
Those seeking an object lesson in the relative value of art and commerce in Hollywood need look no further than Ethan Hawke, who kicked off the summer movie season with Richard Linklater's "Before Midnight" and now brings it to a close with "Getaway," another tale of an American expat winding his way through an exotic foreign locale.
The Reyhanli incident should be an object lesson for Turkey and reactionary states in the region to stop supporting terrorists, the Iranian legislator stated.
WHAT an object lesson in double standards our city council is portraying.
The tale is clearly presented as an object lesson of American immigrant success and as a rebuke to the current atmosphere of xenophobia that surrounds much of the policy debate around immigration.
Bill Morris' compassion and determination to see the victims supported and the perpetrator made accountable through the courts has been an object lesson in proper pastoral response to such crimes.
Once economically successful, Japan, now mired in 15 years of stagnation, should provide an object lesson, after introducing such policies in the 90s.
To Schumer, Solarz would come to serve as an object lesson in all of the wrong choices a politician can make.
When Lance was told by recent graduates of two different police academies they'd been taught to take suspects at gunpoint with fingers on triggers, he went back to an object lesson he had been teaching for many, many years.
Sir Chris is a fantastic role model for all of us, especially the young, and an object lesson to all other aspiring sports persons.