What does it mean?
If people can't learn from your good behaviour, at least they can learn from your mistakes by seeing what not to do.
Who said it?
Catherine Aird: British crime novelist (born 1930). Source: His Burial Too (1973).
Funny– Catherine Aird, His Burial Too (1973)
“If you can't be a good example, then you'll just have to be a horrible warning.”