bad apple
rotten apple

"There are always a few bad apples in any group."

And I'm not going to tell you who that bad apple is, he said to himself.

"You always got one bad apple that turns everything into something else," he said.

"You have some bad apples but generally people are all right."

No doubt it was the mark of a bad apple.

And even if some who now seem perfect end up being particularly bad apples, at least it will make good television.

These have been caused by a few bad apples, which might happen in any new industry.

But not all of them, and everybody knows what a few bad apples can do.

And no, we're not talking about a few bad apples.

It would, they say, hurt everyone because of a few bad apples.