I don't think anyone's mentioned this yet, but from what I can tell, it's easier to be sure about a boy than a girl. Because if you see a little penis then that's a bit of a giveaway, but if you can't get a good view, it could be that you just can't see it.
Which is why I guess I view a boy verdict a bit more confidently than I'd view a girl verdict.
That's what we were always told, buuuuuut....
my sister was told at her 19 week scan (& I think another one after), that she was expecting a boy. By 28 weeks it had changed to a girl. When she asked how they got it wrong the first time they said sometimes the labia can be a bit swollen & look like testicles or a penis.
So she has now gone & bought pink...we'll find out in a few days!