When our daughter was 3, she was playing this little pre-school game on the computer while we were in the next room. I started to hear some singing, so I snuck over to the door so not to disturb our daughter from singing.
As I got to the doorway, I heard the music of Twinkle, Twinkle, Little Star coming from the computer and my daughter was innocently singing her own lyrics. Her lyrics were, "Shit shit, shit shit, shitshit shit, shit shit shitshit shit shit shit. . ."
I started laughing so hard, I was crying and could barely walk, but I did what any good, responsible parent would do. . .I snuck away and got my wife so she could hear it, too. :)
We became extra careful about our language in the house after that.