Ive had 2 friends have this done to them, the 1st friend it was her 1st baby and she got told it was a girl, went out and got everything pink, had a scan at 34 weeks due to baby not moving the amount it should and got told it was a boy, she is lucky that she had the scan and got told so she could go and change everything.
The 2nd friend, it was her 4th baby, she got told it was a girl and she had a few more scans than normal due to problems and found out it was a boy, she hadn't gone out and got pink stuff and didn't buy blue after they got told it was a boy just incase they got it wrong again, she did give birth to a boy though!