When I was little, I was fed the standard "don't talk to strangers" line. When my younger sister was born while I was in kindergarten, I remember sitting in a room with my dad. There was an older lady sitting next to us, and for whatever reason that I can't remember, she gave me a quarter. My dad said "what do you say?", and I stayed quiet because, in my mind, the lady was a stranger and I wasn't supposed to talk to her. So in the end my dad got mad that I didn't thank the lady, and I was left really confused because I didn't know what the hell I was supposed to do.
It's just something that still annoys me whenever I think about it.