I remember this happening to my dad, when I was just a kid. There was a disfellowshipped brother in the cong, whom my dad had been friends with. Before the meeting, my dad just nodded hello to him, didn't even say the word. Shortly after that, he was called aside by an elder and told that it was inappropriate to greet a disfellowshipped person.
I couldn't have been more than 9 or 10 at the time, but it really made an impression on me. I understood what it meant to be disfellowshipped, but couldn't grasp why an innocent, unassuming act like nodding was wrong. Were they communicating telepathically? Was the df'd brother sending evil, apostate thoughts to my father via brain waves? It was all so confusing for my young mind.