lesson from others' mistakes; food for thought
Idiomatic expression meaning that one can learn from the mistakes or failures of others, just as stones from other mountains can be used to polish gems. Often used in the phrase 他山の石とする.
See also: 他山の石以て玉を攻むべし
Let's learn from his mistake and make sure we don't repeat it.
That company's bankruptcy is a lesson for us.