Sex of baby changing at 20 week appointment

So this coming Tuesday we have our 20 week anatomy scan, at our 16 week appointment we found out we were having a boy!! We did a gender reveal and everything but our doctor said she was only 90% sure about it being a boy. I swear her ultrasound machine is the worst and it took her probably 5 min to pick boy or girl! I'm so scared when we go to a better machine they are going to say it's a girl now! Not that I care, I am happy with either! I'm just curious if this has happened to anyone???