Wrong gender

So lately all I've been hearing is stories of mammas who where told there baby was one gender then at birth got the apposite gender. I know usually only once at your big scan you are told the gender of baby, so what's the chances of them being wrong at 3 scans. I had a scan at 15 weeks and was told a girl. She said I would never tell anyone 100% because sometimes we can be wrong but I will say now I'm 90% certain this wee one is a girl. Then at my big scan the midwife said right away yep little girl, then again at my next lol, they all seemed so certain. I've had dreams of a girl too, but because just lately I've heard loads of people say about the opposite being born, I'm having doubts thinking I'm going to have a boy, why else am I hearing all these stories now.