noun
bad example from which one can learn; negative example
Refers to a person or behavior that serves as a lesson by showing what not to do. Often used in educational, workplace, or parenting contexts.
彼の失敗は、私にとって反面教師になった。
His failure became a negative example for me to learn from.
あの上司の態度は反面教師として参考になる。
That boss's attitude is instructive as an example of what not to do.