A negative example from which one can learn; someone or something that serves as a lesson by showing what not to do.
あの経営者の失敗は若い起業家にとって反面教師だ。
That manager's failure is a negative example for young entrepreneurs.
彼の無責任な行動は、クラス全体の反面教師になった。
His irresponsible behavior became a lesson for the whole class on what not to do.
歴史上の失敗は反面教師として繰り返し学ばれるべきだ。
Historical failures should repeatedly be studied as negative examples.