Well it's not happened to me

LOL
I've heard of two women who have had it happen. The one they said it was a girl all teh way through the pregnancy and she had a boy. He's now 3 to give you an idea of how long ago.
The other one was told she was having a boy. Then at her last scan just before she gave birth (like a few days) they told her it was a girl.
LOL that's why I dont entirely trust the scans.