I'm reminded of the Latin phrase "falsus in uno, falsus in omnibus," "false in one, false in all." In Law, it refers to the practice of dismissing a witness' testimony for one thing after they've been shown to be incorrect about another.

In your case, it could be interpreted more like "where there's one thing wrong, there are many things wrong." Just one person acting like this, openly and brazenly, is basically a guarantee there are multiple. Like roaches, or rats. There's never just one.