When I was three or four I asked my Nanna and Gumpy why they didn't get married (Nanna is my mom's widowed mother, Gumpy is my dad's widowed father). It was awfully embarrassing for my parents. It seemed like a perfectly natural assumption to me at the time though - they could share trips, and only drive one car to visit us.

I also have a vague memory of telling a girl from kinder that "I already have this one" when she gave me a barbie for my fourth birthday. My mom gave me a stern talking to after that one, and I have never ever forgotten it. But like people have said, it was not malicious at all - I just didn't know that you weren't supposed to say stuff like that.