How common is an ultrasound to be wrong about the gender ?

A close friend of my family was told she was having a girl but when the baby came it was a boy.... How commen is that??? I'm scared I just found out it's a baby girl last week and I'm so pariniod it will be a boy at birth... Although the ultra sound seemed pretty clear... Legs spread and everything... Should I stop worrying???? I mean everything including dreams and wives tales say girl for me too... Which I know you should go by...