3 lines on a boy ultrasound?

So I have heard stories where woman go in for their gender scans and get told they are having a girl, go back later and known behold it's a boy. Or even "girl" babies being born male. I'm wondering how often does this hold true. I had my scan done last week and they were 100% sure it was a girl, but I don't want to go out and buy all this girl stuff to find out it's a boy later. What do you all think?