Traditionally libel/slander are the obvious examples, but modern examples include identity theft (A takes out a loan from B in the name of C => harm to C), security breaches (A leaks C's password from B's website), "revenge porn", outing people's sexuality without their consent (can get people killed), terrorist radicalisation (A convinces B to murder C), and so on.

"Well, which one? Some communications are harmful to third parties."

A recent example of this are the ability for conspiracy theorists to communicate news stories after these mass shootings that these attacks were staged or the victims are "Fake". This further pushes harmful propaganda that there is not a real threat to students in schools, and the "third party" children are the ones who suffer from this communication.