How did you manage that fine line with your kids when you taught them about "stranger danger," yet didn't want them to get paranoid?

I was at a restaurant today and there was a little girl across from us, about 4, who yelled “HI!” when we sat down. I smiled and waved back.
A bit later I glanced at her and she had whipped cream all over her mouth. It was cute so I grinned at her again, and she grinned back.

It caused me to remember an incident many moons ago, when I took the kid to the pediatrician. We walked in behind a woman and her daughter, who was about 4. The child turned around and looked at me, so I smiled and said, “Hi!”
The girl’s eyes got big and frightened and she pressed herself against her mom and said, “That lady just told me to come here!”
Mom didn’t react or anything, but I thought, “That is a crying shame.” I have no way of knowing if her parents caused this, or if something happened to the poor kid.

So, how did you walk that line?