Gender Mix Up?

What are the chances if gynae checked gender it's a girl yet gave birth its a boy? Went for a week 20 scan too as the report stated its a girl. But there's so many people telling me there will be chances that it will turn out to be a boy (when I've alr bought so many things in Pink and has alr thought if a girl's name). I'm currently week 36 now. Yesterday there's a sales girl even hope that it will turn out to be a boy and told me that right Infront of me. Now I'm getting kinda paranoid