My older sister did the same thing to my younger sister. She thought she was buying a birthday card and got her a mother's day card, this was when we were teens (prior to either of them having any children). The irony is that my younger sister has always had a gift of learning other languages and knew Spanish, so that same year I had bought her a card in Spanish knowing full well that it was a baby's first birthday card but pretended I didn't know what it said and stated "I figured it had a cake on it so it would be appropriate."
So both of her siblings didn't get her a card that made any sense that year.
