The stories are always of a friend of a friend. I have not met anyone in real life who was given the incorrect gender by the sonographer. If they aren't certain they won't tell you. Telling people the wrong thing is a good way to get sued!
Everyone told me that DS2 was going to be a girl as I was a bit sicker, and my boobs didn't get big as early. It is all such rubbish! At least they all stopped after I had the scan.
Don't worry.