Genetic blood test accuracy question

I'm not sure what group to post this in but here it goes... Has anyone ever had a gender blood test and it turned out wrong? I read something that if you've previously had a boy there is something that stays in the mom (I dont know what the name of it is) and it could cause a misreading if you are in fact pregnant again and it's a girl... is that true? Has anyone ever experienced this?

I really hope I'm wording this correctly 🙈