He is truly wise who gains wisdom from another's mishap

From the errors of others, a wise man corrects his own.

He bids fair to grow wise who has discovered that he is not so

Do not turn back when you are just at the goal.

Never thrust your own sickle into another's corn.

One ungrateful man does an injury to all who are suffering. [Lat., Ingratus unus miseris omnibus nocet.]