It hasn't happened to me (not yet at least
) and it's never happened to anyone I know in real life. But, I've heard several stories about it online. I'm not an u/s tech, but from the techs I've learned from online it's been said that a tech should say girl or boy based on the parts they see, not the parts they don't see. To say girl, they should be saying it because they SEE labia. To say boy, they should be saying it because they SEE both penis and scrotum. Otherwise, I believe it may not be as accurate.
For what it's worth, when I was pregnant with my first child, my doctor did the 19 week sono and said the exact same thing your tech said. As it turned out, it was indeed a girl. I never even questioned it at the time because I didn't know anything about u/s during my first pregnancy. If I knew then what I knew now, I would have been skeptical. But, he was right!
Congrats on your pregnancy! 