Wrong gender?

How many of you ladies were told the wrong gender about your baby?
How far along were you when they first told you the gender? And how far along were you when they told you something different?
To make this short, I have a SIL that's always been in an unspoken competition with me, why I have no idea. I have a 10 year old girl, and she has two girls, I am dying to have a boy and I feel like it is, but idk if it's just mind convincing me it is too. Anyways I'm getting a 3D done Monday and I'll be 15+1 then, and she's trying to convince me that they are gonna tell me the wrong thing because it's happened to a lot of people she knows. ? she even had the nerve to say she's hoping I have a girl. Anyways I'm sure it's my hormones cause usually I just let her go in one ear and out the other, but I really want a boy so bad. (AND yes I'll be happy and grateful for a girl too, but there is nothing wrong with preferring to have a certain gender)