Real world idioms: "bad apple," "one bad apple in a barrel spoils all the rest"
I thought of this one, because it's currently being used frequently in the US news. However, I'm going to avoid any reference to current events.
This idiom is very common. Further, it is the best way to describe a certain kind of situation. Unlike other idioms this is one that a foreign speaker <em>can</em> use when it's appropriate. It's not slang, but it is colloquial and informal.
The reference is to a saying, "One bad apple in a barrel spoils all the rest."
This is literally true of apples and other fruit. A common reason why apples spoil and rot is that mold growing on them. Mold spreads. If one apple gets moldy, the apple next to it will get moldy. Eventually they will all get moldy. However, if the moldy apple or apples are spotted quickly and removed, the rest can be saved.
The saying is used figuratively. It is used to refer to corruption or misconduct within an organization. Suppose one person is taking bribes. If he is not stopped, his colleague may decide that bribery is normal, and start taking them too. Soon there is corruption throughout the whole organization.
The "bad apples" metaphor suggests that the organization can be saved, if those in charge can quickly spot and remove the "bad apples."
When an incident of misconduct occurs, those who support the organization, using formal language, might say "We should not judge the organization by the actions of a few." Privately and informally, they might say "So-and-so was a bad apple and should have been fired years ago."
Those who distrust the organization are likely to say "No, this isn't a case of 'a few bad apples.' There is a systemic problem. It can't be fixed by removing the bad apples."