object lesson

noun3

1. •
   A lesson taught (especially to young children) using a familiar or unusual object as a focus.
2. •
   An example from real life that explains a principle or teaches a lesson.

   • Vertebrates are limited to two eyes each, but the variations they have played on this plain vanilla starting point are an object lesson in how much can be made from a little. — 2012
3. •
   Anything used as an example or lesson which serves to warn others as to the outcomes that result from a particular action or behavior, as exemplified by the fates of those who followed that course.

   • Let that be an object lesson to him.