Gender mess up...

Erin
I know that ultrasounds aren't 100% accurate & I've heard stories where parents are told one gender but at the time of delivery are revealed another however...is it more you're told it's a girl but find out its a boy or does it happen the other way around too? 
I can understand if a baby boy is positioned just right u may not see his gender parts so a tech will announce girl.  But can you see a penis & find out later it's a girl? Does the latter happen? 
I've googled & only hear about it being a boy at delivery & was curious if anyone had a different story.